Reset output frames counter for second pass

The frame counter was not being reset at the start of
the first pass.

Change-Id: I2ef7c6edf027e43f83f470c52cbcf95bf152e430

vpxenc.c

@@ -2128,6 +2128,7 @@ static void setup_pass(struct stream_state  *stream,
 
     stream->cx_time = 0;
     stream->nbytes = 0;
+    stream->frames_out = 0;
 }