selinux: handle getcon_raw producing a NULL pointer, despite returning 0

Previously, we assumed that success meant we definitely got a valid
pointer. There is at least one edge case where this is not true (i.e.,
we can get both a 0 return value, and *also* a NULL pointer):
https://github.com/SELinuxProject/selinux/blob/4246bb550dee5246c8567804325b7da206cd76cf/libselinux/src/procattr.c#L175

When this case occurrs, if we don't check the pointer we SIGSEGV in
early initialization.