avformat/utils: Fix invalid NULL pointer operation in ff_parse_key_value()

Fixes: pointer index expression with base 0x000000000000 overflowed to 0xffffffffffffffff
Fixes: 44012/clusterfuzz-testcase-minimized-ffmpeg_dem_HLS_fuzzer-5670607746891776

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

libavformat/utils.c

@@ -1244,7 +1244,7 @@ void ff_parse_key_value(const char *str, ff_parse_key_val_cb callback_get_buf,
         key_len = ptr - key;
 
         callback_get_buf(context, key, key_len, &dest, &dest_len);
-        dest_end = dest + dest_len - 1;
+        dest_end = dest ? dest + dest_len - 1 : NULL;
 
         if (*ptr == '\"') {
             ptr++;