libmpdclient: added assertion on "receiving" flag
libmpdclient: added assertion on "receiving" flag

When the "receiving" flag is true, there must be no error set.

sl0th/libmpdclient,cmende/libmpdclient,jagleeso/libmpdclient,sl0th/libmpdclient,sl0th/libmpdclient,yusiwen/libmpdclient,cmende/libmpdclient,jwise/libmpdclient,cmende/libmpdclient,jagleeso/libmpdclient,psych0tik/libmpdclient2,psych0tik/libmpdclient2,yusiwen/libmpdclient,jwise/libmpdclient,OpenInkpot-archive/iplinux-libmpdclient,jagleeso/libmpdclient,jwise/libmpdclient,yusiwen/libmpdclient,OpenInkpot-archive/iplinux-libmpdclient