ffmpeg: avoid possible undefined behavior

On lines 1633,1634 FFABS(pts) is performed. However, if av_stream_get_end_pts
returns AV_NOPTS_VALUE always, pts remains stuck at INT64_MIN, leading
to undefined behavior on FFABS.

One could conceive of a solution using FFNABS. However, such a solution
has to deal with the implementation defined rounding of integer division
with at least one negative operand in ANSI C89. C99 forces truncation to
zero, but I am not sure that all of our platforms compile with full C99
support, and in particular whether we can safely assume a fixed
rounding behavior across all platforms.

This solution is simple, and I doubt changing INT64_MIN to INT64_MIN + 1
has any practical loss - if it is stuck at its initial value, the stream
is messed up anyway.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
