8007312: null check signal semaphore in os::signal_notify windows

Reviewed-by: dholmes, sla

src/os/windows/vm/os_windows.cpp

@@ -1940,7 +1940,7 @@ int os::sigexitnum_pd(){
 
 // a counter for each possible signal value, including signal_thread exit signal
 static volatile jint pending_signals[NSIG+1] = { 0 };
-static HANDLE sig_sem;
+static HANDLE sig_sem = NULL;
 
 void os::signal_init_pd() {
   // Initialize signal structures
@@ -1970,10 +1970,11 @@ void os::signal_init_pd() {
 
 void os::signal_notify(int signal_number) {
   BOOL ret;
-
+  if (sig_sem != NULL) {
     Atomic::inc(&pending_signals[signal_number]);
     ret = ::ReleaseSemaphore(sig_sem, 1, NULL);
     assert(ret != 0, "ReleaseSemaphore() failed");
+  }
 }
 
 static int check_pending_signals(bool wait_for_signal) {