fix null pointer dereference introduced in last sigprocmask commit
fix null pointer dereference introduced in last sigprocmask commit

chneukirchen/musl-chris2,davidlazar/musl,chardi/musl,chardi/musl,rofl0r/musl-old,davidlazar/musl,rofl0r/musl-old,chardi/musl,chneukirchen/musl-chris2