avcodec/imm4: Unreference previous frame on frame size change

Fixes: Out of array access
Fixes: 13552/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_IMM4_fuzzer-5767949648920576

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

libavcodec/imm4.c

@@ -446,11 +446,13 @@ static int decode_frame(AVCodecContext *avctx, void *data,
         return AVERROR_PATCHWELCOME;
     }
 
-    if (!frame->key_frame &&
-        (avctx->width != width ||
-         avctx->height != height)) {
-        av_log(avctx, AV_LOG_ERROR, "Frame size change is unsupported.\n");
-        return AVERROR_INVALIDDATA;
+    if (avctx->width  != width ||
+        avctx->height != height) {
+        if (!frame->key_frame) {
+            av_log(avctx, AV_LOG_ERROR, "Frame size change is unsupported.\n");
+            return AVERROR_INVALIDDATA;
+        }
+        av_frame_unref(s->prev_frame);
     }
 
     ret = ff_set_dimensions(avctx, width, height);