cache: exit early on cancellation

An approximate measure to make it exit possibly slightly earlier.
Relatively speaking, some time will pass between cancellation and
the cache actually being requested to exit, so it's good if the
cache returns EOF immediately.

stream/cache.c

@@ -208,6 +208,9 @@ static bool cache_fill(struct priv *s)
             goto done;
     }
 
+    if (mp_cancel_test(s->cache->cancel))
+        goto done;
+
     // number of buffer bytes which should be preserved in backwards direction
     int64_t back = MPCLAMP(read - s->min_filepos, 0, s->back_size);