avcodec/h264_metadata_bsf: Fix for the incorrect user data with hyphens

avcodec/h264_metadata_bsf: Fix for the incorrect user data with hyphens

How to reproduce:
./ffmpeg -f lavfi -i testsrc -c:v libx264 -g 25 -bsf:v h264_metadata=sei_user_data=186f3693-b7b3-4f2c-9653-21492feee5b8+hello -frames:v 1 h264.mp4

master:
[Parsed_showinfo_0 @ 0x7fc8a0703180] UUID=186f3693-7030-4f2c-6030-21492feee5b8
[Parsed_showinfo_0 @ 0x7fc8a0703180] User Data=hello

Applied the patch:
[Parsed_showinfo_0 @ 0x7f969d408e00] UUID=186f3693-b7b3-4f2c-9653-21492feee5b8
[Parsed_showinfo_0 @ 0x7f969d408e00] User Data=hello

Signed-off-by: Limin Wang <lance.lmwang@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>