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>
(cherry picked from commit 59328aab)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>