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

diff --git a/vpxenc.c b/vpxenc.c
index 059cdd5..d32b21b 100644
--- a/vpxenc.c
+++ b/vpxenc.c
@@ -2128,6 +2128,7 @@
 
     stream->cx_time = 0;
     stream->nbytes = 0;
+    stream->frames_out = 0;
 }