ffprobe: always exit 1 in case of errors

This is consistent with the other ff* tools, and also avoids spurious
success reports when ret%256 = 0.

ffprobe.c

@@ -2359,5 +2359,5 @@ end:
 
     avformat_network_deinit();
 
-    return ret;
+    return ret < 0;
 }