avformat/aviobuf: Keep checksum_ptr consistent in avio_seek()

Otherwise it might be > buf_ptr in which case ffio_get_checksum()
could segfault (s->buf_ptr - s->checksum_ptr would be negative
which would be converted to something very big when converted
to unsigned for the update_checksum callback).

Fixes ticket #11233.

Reported-by: Du4t
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
(cherry picked from commit 987c955cd7e972d9940284fa6ae7187ac858ebb1)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/aviobuf.c

@@ -355,7 +355,7 @@ int64_t avio_seek(AVIOContext *s, int64_t offset, int whence)
         ctx->seek_count++;
         if (!s->write_flag)
             s->buf_end = s->buffer;
-        s->buf_ptr = s->buf_ptr_max = s->buffer;
+        s->checksum_ptr = s->buf_ptr = s->buf_ptr_max = s->buffer;
         s->pos = offset;
     }
     s->eof_reached = 0;