PR gdb/21164: maint print {symbols,msymbols,psymbols} without args crash

PR gdb/21164: maint print {symbols,msymbols,psymbols} without args crash

This is a fix for PR gdb/21164.  The problem started to happen after:

 commit 34c41c681f4a0a0dfe0405c7d2aecf458520557a
 Author:     Doug Evans <xdje42@gmail.com>
 AuthorDate: Mon Dec 19 08:33:46 2016 -0800

    New syntax for mt print symbols,msymbols,psymbols.

This change introduced new syntax for the mentioned commands, and
improved the parsing of arguments by using 'gdb_buildargv'.  However,
it is necessary to check if the argv being built is not NULL, which
can happen if the user doesn't provide any arguments to these
commands.

gdb/ChangeLog:
2017-02-15  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR gdb/21164
	* psymtab.c (maintenance_print_psymbols): Verify if 'argv' is not
	NULL before using it.
	* symmisc.c (maintenance_print_symbols): Likewise.
	(maintenance_print_msymbols): Likewise.

gdb/testsuite/ChangeLog:

gdb/ChangeLog:
2017-02-15  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR gdb/21164
	* gdb.base/maint.exp: Add testcases for when the commands do
	not have arguments.