Fix completing an empty string

Fix completing an empty string

Earlier while working on the big completer rework series, I managed to
break

 (gdb) [TAB]

locally, and make GDB crash, but only notice a few weeks down the
road, because we have no test for that...

I also noticed that:

 (gdb)     [TAB]

didn't work (didn't show all commands as matches), even though
entering a command with leading whitespace works:

 (gdb)     help

This commit fixes the latter and adds a testcase that covers both
issues.

The gdb.base/completion.exp change is necessary because the new test
has a file name that also starts with "gdb.base/complet", making that
particular test ambiguous.  Adding another letter disambiguates.

gdb/ChangeLog:
2017-11-25   Pedro Alves  <palves@redhat.com>

	* completer.c (complete_line_internal_1): Skip spaces until the
	start of the command.

gdb/testsuite/ChangeLog:
2017-11-25   Pedro Alves  <palves@redhat.com>

	* gdb.base/complete-empty.exp: New file.
	* gdb.base/completion.exp: Adjust.