Fix latent bug with custom word point completers

Fix latent bug with custom word point completers

Completion routines that use a custom word point, and that then
recurse into complete_line (e.g., if we make "thread apply" a custom
word point completer, and complete on the command passed as argument),
we stumble on this latent bug:

 (gdb) thread apply all pri[TAB]
 (gdb) thread apply all priprint

The problem is that there's a spot in complete_line_internal_1 that
rewinds the completion word but does not reflect that change in the
custom word point in the tracker.  This patch fixes it.

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

	* completer.c (complete_line_internal_1): Rewind completion word
	point.
	(completion_tracker::advance_custom_word_point_by): Change
	parameter type to int.
	* completer.h (completion_tracker::advance_custom_word_point_by):
	Likewise.