avformat/utils: Discard huge timestamps which would cause overflows if used in basic...

avformat/utils: Discard huge timestamps which would cause overflows if used in basic computations

Allowing larger timestamps makes it impossible to calculate basic things like the
difference of 2 timestamps or their sum without checking each individual computation for
overflow.
This should avoid a significant number of overflow checks

Fixes Ticket5136

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>