lwp: Fix signal wait screwup for multiple different signals

lwpSigWait() clears LwpSigCheck even when signals remain in
LwpSigCaught.  The next empth_wait_for_signal() will then wait until
another one gets caught.  Broken in commit fe2de3d74, v4.3.10.

Fix by clearing it only when LwpSigCaught is empty.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>