lavf: Only initialize s->offset once when using avoid_negative_ts make_zero

When given a stream starting at dts=0, it would previously consider
s->offset as uninitialized and set an offset when the second packet
was written, ending up writing two packets with dts=0. By initializing
this field to AV_NOPTS_VALUE, we make sure that we only initialize it
once, on the first packet.

Signed-off-by: Martin Storsjö <martin@martin.st>