posix: Declare Group.gr_mem as null-terminated
posix: Declare Group.gr_mem as null-terminated

Fixes bug 725149.

spidermonkdust/vala,spidermonkdust/vala,frida/vala,GNOME/vala,rvu95/vala,GNOME/vala,flobrosch/vala-fixes,rvu95/vala,frida/vala,spidermonkdust/vala,rvu95/vala,flobrosch/vala-fixes,GNOME/vala,frida/vala,flobrosch/vala-fixes,GNOME/vala,frida/vala,frida/vala