CRASH app calling ppoll with NULL sigmask

This is a bug in 69ea4ef0 for #2759. The handling of ppoll (and probably the other p* sigmask syscalls) does not gracefully handle a NULL sigmask: the code just crashes. NULL is a valid parameter:

   If the sigmask argument is specified as NULL, then no signal mask
   manipulation is performed (and thus ppoll() differs from poll() only
   in the precision of the timeout argument).

Even if it were not, DR should gracefully handle invalid parameters and should not crash on any app behavior.