services_discovery: playlist_NodesPairCreate needs a lock.
services_discovery: playlist_NodesPairCreate needs a lock.

Should fix #1708 (Adding a folder in the Mac OS X VLC *media library* result in an assertion failure). Thanks to upcoming commit "playlist: Use PL_ASSERT_LOCKED where the playlist lock should be held.".

shyamalschandra/vlc,krichter722/vlc,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.2,shyamalschandra/vlc,shyamalschandra/vlc,vlc-mirror/vlc,vlc-mirror/vlc,vlc-mirror/vlc,shyamalschandra/vlc,xkfz007/vlc,jomanmuk/vlc-2.1,xkfz007/vlc,jomanmuk/vlc-2.1,krichter722/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,shyamalschandra/vlc,krichter722/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc-2.1,krichter722/vlc,krichter722/vlc,vlc-mirror/vlc,xkfz007/vlc,krichter722/vlc,krichter722/vlc,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1,shyamalschandra/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc,xkfz007/vlc,jomanmuk/vlc-2.1,xkfz007/vlc,xkfz007/vlc,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.2,xkfz007/vlc