avformat/4xm: Cleanup on GET_LIST_HEADER() failure

Fixes: memleak
Fixes: 23142/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5932860820422656

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/4xm.c

@@ -59,8 +59,10 @@
 #define GET_LIST_HEADER() \
     fourcc_tag = avio_rl32(pb); \
     size       = avio_rl32(pb); \
-    if (fourcc_tag != LIST_TAG) \
-        return AVERROR_INVALIDDATA; \
+    if (fourcc_tag != LIST_TAG) { \
+        ret = AVERROR_INVALIDDATA; \
+        goto fail; \
+    } \
     fourcc_tag = avio_rl32(pb);
 
 typedef struct AudioTrack {
@@ -210,7 +212,7 @@ static int fourxm_read_header(AVFormatContext *s)
     unsigned int size;
     int header_size;
     FourxmDemuxContext *fourxm = s->priv_data;
-    unsigned char *header;
+    unsigned char *header = NULL;
     int i, ret;
 
     fourxm->track_count = 0;