Fix a crash in vpxdec

If the codec fails to return a frame on the second frame (as in a VP8 alt-ref,
then img will be a null pointer and dereferencing it while trying to write
the color format (again) will crash.

Change-Id: Ie48710d4c5715f532d453c879c71c20ccdeb81cc

vpxdec.c

@@ -1025,7 +1025,7 @@ int main(int argc, const char **argv_) {
       show_progress(frame_in, frame_out, dx_time);
 
     if (!noblit) {
-      if (frame_out == 1 && use_y4m) {
+      if (frame_out == 1 && img && use_y4m) {
         /* Write out the color format to terminate the header line */
         const char *color =
             img->fmt == VPX_IMG_FMT_444A ? "C444alpha\n" :