avcodec/rv34: Fix runtime error: signed integer overflow: -2 + -2147483648 cannot be represented in type 'int'

Fixes: 642/clusterfuzz-testcase-558358808074649

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

libavcodec/rv34.c

@@ -866,6 +866,11 @@ static int rv34_decode_mv(RV34DecContext *r, int block_type)
     for(i = 0; i < num_mvs[block_type]; i++){
         r->dmv[i][0] = get_interleaved_se_golomb(gb);
         r->dmv[i][1] = get_interleaved_se_golomb(gb);
+        if (r->dmv[i][0] == INVALID_VLC ||
+            r->dmv[i][1] == INVALID_VLC) {
+            r->dmv[i][0] = r->dmv[i][1] = 0;
+            return AVERROR_INVALIDDATA;
+        }
     }
     switch(block_type){
     case RV34_MB_TYPE_INTRA: