util: fix u_fifo_pop()

Seems like no one ever depended on it to actually return false when fifo
is empty.

Fixes: 6e61d06209 ("util: Add super simple fifo")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4366>

src/gallium/auxiliary/util/u_fifo.h

@@ -80,7 +80,7 @@ u_fifo_pop(struct util_fifo *fifo, void **ptr)
 
    *ptr = array[fifo->tail];
 
-   ++fifo->num;
+   --fifo->num;
 
    return TRUE;
 }