stat: fix printing selinux context and null-dereference

busybox stat tries to always print the selinux context, even if it
is not requested which leads to a segmentation fault due to
dereferencing a null-pointer.

This also changes the format-string used to print the context to
so it actually produces useful output.

Change-Id: Idfe15c931840457f357f860e560fca5b0480ad6f