Correct the advancement of a pointer to skip over LFE samples as
intended in the loudness computation.

This suggested patch is 3 of 3 for Ticket #2144 "libavfilter ebur128
loudness inaccuracy, irregular time interval, LFE interference".
This suggested patch supersedes the contribution of January 15, 2013
in the thread “[PATCH] lavfi: EBU R.128 irreg time, ch weights, skip
LFE” which combined multiple fixes in one batch.