selinux: keep context struct members in sync

Commit 53f3517a ("selinux: do not leave dangling pointer behind")
reset the `str` field of the `context` struct in an OOM error branch.
In this struct the fields `str` and `len` are coupled and should be kept
in sync.  Set the length to zero according to the string be set to NULL.

Fixes: 53f3517a ("selinux: do not leave dangling pointer behind")
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/ss/context.h

@@ -168,6 +168,7 @@ static inline int context_cpy(struct context *dst, const struct context *src)
 	if (rc) {
 		kfree(dst->str);
 		dst->str = NULL;
+		dst->len = 0;
 		return rc;
 	}
 	return 0;