lavf: fix signed overflow in avformat_find_stream_info()

On the first iteration through this code, last_dts is always
INT64_MIN (AV_NOPTS_VALUE) and the subtraction overflows in
an invalid manner.  Although the result is only used if the
input values are valid, performing the subtraction is still
not allowed in a strict environment.

Signed-off-by: Mans Rullgard <mans@mansr.com>