Avoid the memory leak in the add_job function

1. When allocating memory for future, if the allocation fails, do not
simply return NULL. Instead, release the memory allocated for job
beforehand to avoid memory leaks.

Author: weihsinyeh

examples/Makefile

@@ -3,4 +3,4 @@ all:
 clean:
 	rm -f rmw_example
 check: all
-	./rmw_example
+	./rmw_example

examples/rmw_example.c

@@ -191,8 +191,10 @@ struct tpool_future *add_job(tpool_t *thrd_pool, void *(*func)(void *),
         return NULL;
 
     struct tpool_future *future = tpool_future_create();
-    if (!future)
+    if (!future){
+        free(job);
         return NULL;
+    }
 
     // unprotected producer
     job->args = args;