avcodec/mimic: Use ff_set_dimensions() to set the dimensions

Fixes: OOM
Fixes: 1671/clusterfuzz-testcase-minimized-4759078033162240

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

libavcodec/mimic.c

@@ -390,9 +390,11 @@ static int mimic_decode_frame(AVCodecContext *avctx, void *data,
             return AVERROR_INVALIDDATA;
         }
 
+        res = ff_set_dimensions(avctx, width, height);
+        if (res < 0)
+            return res;
+
         ctx->avctx     = avctx;
-        avctx->width   = width;
-        avctx->height  = height;
         avctx->pix_fmt = AV_PIX_FMT_YUV420P;
         for (i = 0; i < 3; i++) {
             ctx->num_vblocks[i] = AV_CEIL_RSHIFT(height,   3 + !!i);