avformat/img2dec: Fix probe_buffer leak in ff_img_read_header()

Fixes: memleak
Fixes: 15171/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5683881644130304

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

libavformat/img2dec.c

@@ -356,6 +356,7 @@ int ff_img_read_header(AVFormatContext *s1)
             }
             if (s1->flags & AVFMT_FLAG_CUSTOM_IO) {
                 avio_seek(s1->pb, 0, SEEK_SET);
+                av_freep(&probe_buffer);
             } else
                 ffio_rewind_with_probe_data(s1->pb, &probe_buffer, probe_buffer_size);
         }