avcodec/xpmdec: Check size before allocation to avoid truncation

Fixes:OOM
Fixes:out of array access (no testcase)
Fixes: 48567/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_XPM_fuzzer-6573323838685184

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 95f0f84dae)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/xpmdec.c

@@ -354,6 +354,9 @@ static int xpm_decode_frame(AVCodecContext *avctx, AVFrame *p,
         return AVERROR_INVALIDDATA;
     }
 
+    if (size > SIZE_MAX / 4)
+        return AVERROR(ENOMEM);
+
     size *= 4;
 
     ptr += mod_strcspn(ptr, ",") + 1;