avcodec/error_resilience: Use a symmetric check for skipping MV estimation

This speeds up the testcase by a factor of 4

Fixes: Timeout
Fixes: 13100/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMV2_fuzzer-5767533905313792

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/error_resilience.c

@@ -437,7 +437,7 @@ static void guess_mv(ERContext *s)
     }
 
     if ((!(s->avctx->error_concealment&FF_EC_GUESS_MVS)) ||
-        num_avail <= mb_width / 2) {
+        num_avail <= FFMAX(mb_width, mb_height) / 2) {
         for (mb_y = 0; mb_y < mb_height; mb_y++) {
             for (mb_x = 0; mb_x < s->mb_width; mb_x++) {
                 const int mb_xy = mb_x + mb_y * s->mb_stride;