st/mesa: early exit on error path

Can't rely on asserts having any effect on flowcontrol for release
builds.

src/mesa/state_tracker/st_cb_condrender.c

@@ -69,6 +69,7 @@ st_BeginConditionalRender(GLcontext *ctx, struct gl_query_object *q,
       break;
    default:
       assert(0 && "bad mode in st_BeginConditionalRender");
+      return;
    }
 
    pipe->render_condition(pipe, stq->pq, m);