Fix printf of a convenience variable holding an inferior address

Fix printf of a convenience variable holding an inferior address

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commit 1f6f6e21fa86dc3411a6498608f32e9eb24b7851
Author: Philippe Waroquiers <philippe.waroquiers@skynet.be>
Date:   Mon Jun 10 21:41:51 2019 +0200

    Ensure GDB printf command can print convenience var strings without a target.

GDB was extended in order to allow the printing of convenience
variables that are strings without a target.  However, this introduced
a regression that hasn't been caught by our testsuite (because there
were no tests for it).

The problem happens when we try to print a convenience variable that
holds the address of a string in the inferior.  The following
two-liners can reproduce the issue:

$ echo -e 'int main(){const char a[]="test";return 0;}' | gcc -x c - -O0-g3
$ ./gdb/gdb --data-directory ./gdb/data-directory -q ./a.out -ex 'start' -ex 'set $x = (const char *) (&a[0] + 2)' -ex 'printf "%s\n", $x'

After some investigation, I found that the problem happens on
printcmd.c:printf_c_string.  In the case above, we're taking the first
branch of the 'if' condition, which assumes that there will be a value
to be printed at "value_contents (value)".  There isn't.  We actually
need to obtain the address that the variable points to, and read the
contents from memory.

It seems to me that we should avoid this branch if the TYPE_CODE of
"value_type (value)" is TYPE_CODE_PTR (i.e., a pointer to the
inferior's memory).  This is what this patch does.

I took the liberty to extend the current testcase under
gdb.base/printcmds.exp and create a test that exercises this scenario.

No regressions have been found on Buildbot.

gdb/ChangeLog:
2020-03-03  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR gdb/25650
	* printcmd.c (print_c_string): Check also for TYPE_CODE_PTR
	when verifying if dealing with a convenience variable.

gdb/testsuite/ChangeLog:
2020-03-03  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR gdb/25650
	* gdb.base/printcmds.exp: Add test to verify printf of a
	variable holding an address.