8182034: os::Posix::is_valid_signal() should, if it uses sigaddset, initialize the signal set

Added missing sigemptyset.

Reviewed-by: kbarrett, clanger

hotspot/src/os/posix/vm/os_posix.cpp

@@ -760,6 +760,7 @@ bool os::Posix::is_valid_signal(int sig) {
 #else
   // Use sigaddset to check for signal validity.
   sigset_t set;
+  sigemptyset(&set);
   if (sigaddset(&set, sig) == -1 && errno == EINVAL) {
     return false;
   }