avcodec/exif/exif_add_metadata: add support for SSHORT & SBYTE

No working testcase, this omission was just spoted when the parser apparently went out of
sync.

Reviewed-by; Thilo Borgmann <thilo.borgmann@mail.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/exif: do not follow 0 offsets

Fixes exif decoding failure of 1295328_300.jpg

Reviewed-by; Thilo Borgmann <thilo.borgmann@mail.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>