avutil/timecode: Fix fps check

The fps variable is explicitly set to -1 in case of some errors, the check must
thus be signed or the code setting it needs to use 0 as error code
the type of the field could be changed as well but its in an installed header

Fixes: integer overflow
Fixes: 9982cc157b1ea90429435640a989122f/asan_generic_3ad004a_3799_22cf198d9cd09928e2d9ad250474fa58.mov

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
