Bug#45288: pb2 returns a lot of compilation warnings on linux

Fix warnings flagged by the new warning option -Wunused-but-set-variable
that was added to GCC 4.6 and that is enabled by -Wunused and -Wall. The
option causes a warning whenever a local variable is assigned to but is
later unused. It also warns about meaningless pointer dereferences.

storage/example/ha_example.cc

@@ -153,7 +153,7 @@ static int example_done_func(void *p)
   hash_free(&example_open_tables);
   pthread_mutex_destroy(&example_mutex);
 
-  DBUG_RETURN(0);
+  DBUG_RETURN(error);
 }