avformat/rtpenc: Fix integer overflow in NTP_TO_RTP_FORMAT

RTCP synchronization packet was broken since commit in ffmpeg version > 2.8.3
(commit: e04b039b) Since this commit (2e814d03)
"rtpenc: Simplify code by introducing a macro for rescaling NTP timestamps", NTP_TO_RTP_FORMAT
uses av_rescale_rnd() function to add the data to the packet.

This causes an overflow in the av_rescale_rnd() function and it will return INT64_MIN.
Causing the NTP stamp in the RTCP packet to have an invalid value.

Github: Closes #182

Reverting commit '2e814d03' solves the problem.