Don't handle signals in non-HPHP threads

This is not correct, as we are dropping signals because
they got sent to non-HPHP threads.

hphp/runtime/ext/ext_process.cpp

@@ -277,7 +277,7 @@ public:
 IMPLEMENT_STATIC_REQUEST_LOCAL(SignalHandlers, s_signal_handlers);
 
 static void pcntl_signal_handler(int signo) {
-  if (signo > 0 && signo < _NSIG) {
+  if (signo > 0 && signo < _NSIG && !g_context.isNull()) {
     s_signal_handlers->signaled[signo] = 1;
     RequestInjectionData &data = ThreadInfo::s_threadInfo.getNoCheck()->
                                    m_reqInjectionData;