gdb: fix crashes and weird output from new boxed hint text

gdb: fix crashes and weird output from new boxed hint text

After the commit:

  commit f6df8aa48f120b78f0670b429f8a3363020a47dc
  Date:   Mon Sep 15 11:56:17 2025 -0300

      gdb: Make startup message more user friendly

I noticed, that when I start GDB with a file on the command line, I
was seeing some stray '..'.  Like this:

  $ gdb -nw -nh /tmp/hello.x
  GNU gdb (GDB) 18.0.50.20251202-git
  Copyright (C) 2025 Free Software Foundation, Inc.
  License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
  This is free software: you are free to change and redistribute it.
  There is NO WARRANTY, to the extent permitted by law.
  Type "show copying" and "show warranty" for details.
  This GDB was configured as "x86_64-pc-linux-gnu".
  Type "show configuration" for configuration details.
  For bug reporting instructions, please see:
  <https://www.gnu.org/software/gdb/bugs/>.

  ┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
  ┃ Find the GDB manual online at:                                               ┃
  ┃ http://www.gnu.org/software/gdb/documentation/.                              ┃
  ┃ For help, type "help".                                                       ┃
  ┃ Type "apropos <word>" to search for commands related to <word>               ┃
  ┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┛
  ..
  Reading symbols from /tmp/hello.x...

Notice the '..' after the boxed hint text, that's what I'm complaining
about.  Also, notice that the last line within the box is missing its
period.

Before the above commit the last line would appear like this when no
file was loaded:

  Type "apropos <word>" to search for commands related to <word>.

And like this when a file was being loaded:

  Type "apropos <word>" to search for commands related to <word>...

The extra '..' are added to show that a file is being loaded, and that
this might take some time.  But we have the 'Reading symbols from ...'
text to indicate this now, so I think the extra '..' are redundant.
Lets just drop them.  This will leave just a single period at the end
of the sentence.

The above commit unfortunately, didn't include any tests, so I thought
I'd write some to cover this fix.... and that uncovered a bug where
the box around the startup hints could be corrupted:

  $ gdb -eiex 'set width 50' -nw -nh
  GNU gdb (GDB) 18.0.50.20251202-git
  Copyright (C) 2025 Free Software Foundation, Inc.
  License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
  This is free software: you are free to change and redistribute it.
  There is NO WARRANTY, to the extent permitted by law.
  Type "show copying" and "show warranty" for details.
  This GDB was configured as "x86_64-pc-linux-gnu".
  Type "show configuration" for configuration details.
  For bug reporting instructions, please see:
  <https://www.gnu.org/software/gdb/bugs/>.

  ┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
  ┃ Find the GDB manual online at:                 ┃
  ┃ http://www.gnu.org/software/gdb/documentation/. ┃
  ┃ For help, type "help".                         ┃
  ┃ Type "apropos <word>" to                       ┃
  ┃  search for commands related to <word>         ┃
  ┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┛

  (gdb)

This was caused by a mistake on the line where we choose whether to
box or not.  The line is currently:

  if (width - 3 <= docs_url.length ())

There are two problems here, the '3' should be '4'.  The box adds 4
characters '| ' and ' |'.  But also, the WIDTH can be very small, less
than 4 even, which means that the subtraction can underflow, wrapping
around and giving a very large value.  I plan to rewrite the line to:

  if (width < docs_url.length () + 1 + 4)

The '+ 1' accounts for the period at the end of the URL line (which
was previously handled by the '<=', and the '+ 4' accounts for the box
borders.  By making it a '+ 4' on the URL, rather than '- 4' from the
width, we avoid underflow.  This is fine so long as the URL to our
documentation doesn't approach UINT_MAX in length.  Which I hope it
never does.

I've added a couple of asserts to print_gdb_hints to reflect things
that must be true.  The first is that get_chars_per_line never returns
0.  And later on, I assert that 'width > 4' in a place where we are
about to do 'width - 4'.  If the assert triggers then underflow would
have occurred.

Reviewed-By: Guinevere Larsen <guinevere@redhat.com>
Approved-By: Tom Tromey <tom@tromey.com>