Fix latent bug in custom word point completion handling

Fix latent bug in custom word point completion handling

Without this fix, if we switch the "print" completer to custom word
point handling, we regress gdb.base/completion.exp like this:

 (gdb) p "break1.c FAIL: gdb.base/completion.exp: complete 'p "break1' (timeout)

The problem is that completing an expression that starts with double
quotes, and resolves to a filename, like this:

 (gdb) p "break1[TAB]

would change from this, with current master:

 (gdb) p "break1.c"|
         ^^^^^^^^^^|
                   \- cursor here

to this:

 (gdb) p "break1.c |
         ^^^^^^^^^^|
                   \- quote replaced by space

The issue is that completer.c:advance_to_completion_word misses
telling the completion tracker to emulate readline's handling of
completing a string when rl_find_completion_word returns a delimiter.

This commit fixes the latent bug.

gdb/ChangeLog:
2019-06-13  Pedro Alves  <palves@redhat.com>

	* completer.c (advance_to_completion_word): Handle delimiters.