gdb: Fix bug with dbx style func command.

gdb: Fix bug with dbx style func command.

The func command, available when starting gdb in dbx mode, is supposed
to take a function name and locate the frame for that function in the
stack.  This has been broken for a while due to an invalid check of the
arguments within the worker function.  Fixed in this commit.

gdb/ChangeLog:

	* stack.c (func_command): Return early when there is no ARG
	string.

gdb/testsuite/ChangeLog:

	* gdb.base/dbx.exp (test_func): Remove xfails, update expected
	results.