vpxdec: ignore GET_FRAME_CORRUPTED error w/--keep-going

this function may return an error if no frame is available; --keep-going
is meant to test decoder resilience, so simply warn in this case.

Change-Id: I6e6aed3e78eca21cca80d7d8a06a1a244685ba29

vpxdec.c

@@ -908,6 +908,7 @@ int main_loop(int argc, const char **argv_) {
 
     if (vpx_codec_control(&decoder, VP8D_GET_FRAME_CORRUPTED, &corrupted)) {
       warn("Failed VP8_GET_FRAME_CORRUPTED: %s", vpx_codec_error(&decoder));
+      if (!keep_going)
         goto fail;
     }
     frames_corrupted += corrupted;