retry_transfer_wrapper: only call interrupt callback when less than requested is r/w.

Its a waste of cpu cycles otherwise

libavformat/avio.c

@@ -237,7 +237,7 @@ static inline int retry_transfer_wrapper(URLContext *h, unsigned char *buf, int
         if (ret)
            fast_retries = FFMAX(fast_retries, 2);
         len += ret;
-        if (url_interrupt_cb())
+        if (len < size && url_interrupt_cb())
             return AVERROR_EXIT;
     }
     return len;