[obs] fix another potential deadlock

VERSION

@@ -1 +1 @@
-B1-186-g7a96642498+1
+B1-187-gcfd8126e5d+1

obs/lg.c

@@ -199,6 +199,7 @@ static void lgVideoTick(void * data, float seconds)
 
     printf("lgmpClientProcess: %s\n", lgmpStatusString(status));
     this->state = STATE_STOPPING;
+    os_sem_post(this->frameSem);
     return;
   }