Fixed clang-analyzer issue with calling schedule_imm with the continuation

possibly being a nullptr

Author: bryancall

proxy/http2/Http2Stream.cc

@@ -92,11 +92,13 @@ Http2Stream::main_event_handler(int event, void *edata)
     inactive_timeout_at = Thread::get_hrtime() + inactive_timeout;
     if (e->cookie == &write_vio) {
       if (write_vio.mutex) {
-        MUTEX_TRY_LOCK(lock, write_vio.mutex, this_ethread());
-        if (lock.is_locked() && write_vio.cont && this->current_reader) {
-          write_vio.cont->handleEvent(event, &write_vio);
-        } else {
-          this_ethread()->schedule_imm(write_vio.cont, event, &write_vio);
+        if (write_vio.cont && this->current_reader) {
+          MUTEX_TRY_LOCK(lock, write_vio.mutex, this_ethread());
+          if (lock.is_locked()) {
+            write_vio.cont->handleEvent(event, &write_vio);
+          } else {
+            this_ethread()->schedule_imm(write_vio.cont, event, &write_vio);
+          }
         }
       }
     } else {