selinux: do not leave dangling pointer behind

In case mls_context_cpy() fails due to OOM set the free'd pointer in
context_cpy() to NULL to avoid it potentially being dereferenced or
free'd again in future.  Freeing a NULL pointer is well-defined and a
hard NULL dereference crash is at least not exploitable and should give
a workable stack trace.

Fixes: 12b29f3455 ("selinux: support deferred mapping of contexts")
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/ss/context.h

@@ -167,6 +167,7 @@ static inline int context_cpy(struct context *dst, const struct context *src)
 	rc = mls_context_cpy(dst, src);
 	if (rc) {
 		kfree(dst->str);
+		dst->str = NULL;
 		return rc;
 	}
 	return 0;