Return early if callback_only is not used on a function

There was a missing return statement in the attribute handler when
checking whether or not the attribute was used on a function.  Since
positional_argument was introduced into the handler, this uncaught error
would result in an ICE.  This patch fixes that.

gcc/c-family/ChangeLog:

	* c-attribs.cc (handle_callback_only_attribute): Add early
	return if the attribute is not used on a function.

gcc/testsuite/ChangeLog:

	* gcc.dg/attr-callback.c: Add testcases for the early return.

Signed-off-by: Josef Melcr <josef.melcr@suse.com>