avcodec/imm4: Use ff_set_dimensions()

Fixes: Out of memory
Fixes: 10970/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_IMM4_fuzzer-5698750043914240

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/imm4.c

@@ -428,8 +428,9 @@ static int decode_frame(AVCodecContext *avctx, void *data,
             av_log(avctx, AV_LOG_ERROR, "Frame size change is unsupported.\n");
             return AVERROR_INVALIDDATA;
         }
-        avctx->width = width;
-        avctx->height = height;
+        ret = ff_set_dimensions(avctx, width, height);
+        if (ret < 0)
+            return ret;
     }
 
     s->changed_size = 1;