During a resampling operation where

1) user has specified first_pts
2) SWR_FLAG_RESAMPLE is not set initially (directly or otherwise)
3) first_pts has been fulfilled (always using hard compensation)

then upon first encountering a delay where a soft compensation is
required, swr_set_compensation will lead to another init of swr which
will reset outpts to the specified firstpts thus leading to an output
frame having its pts = firstpts. When the next input frame is received,
swr will see a large delay and inject silence from firstpts to the
current frame's pts. This can lead to severe desync and in worst case,
loss of audio playback.

Parameter firstpts initialized to AV_NOPTS_VALUE in swr_alloc and then
checked in swr_init to avoid resetting outpts, thus avoiding reapplication
of firstpts.

Fixes #4131.