[core] Added missing RCV buffer lock in CUDT::receiveBuffer(..).
[core] Added missing RCV buffer lock
in CUDT::receiveBuffer(..).

Co-authored-by: Maxim Sharabayko <5463bd9000ea4f51bf345aa4d6d41eeef1a8aa65@haivision.com>

ethouris/srt,Cinegy/srt,ethouris/srt,Haivision/srt,ethouris/srt,ethouris/srt,Cinegy/srt,Haivision/srt,Haivision/srt,Haivision/srt,Cinegy/srt,Cinegy/srt