livepatch: robustify klp_register_patch() API error checking

Commit 425595a7 ("livepatch: reuse module loader code to write
relocations") adds a possibility of dereferncing pointers supplied by the
consumer of the livepatch API before sanity (NULL) checking them (patch
and patch->mod).

Spotted by smatch tool.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Jessica Yu <jeyu@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

kernel/livepatch/core.c

@@ -876,6 +876,9 @@ int klp_register_patch(struct klp_patch *patch)
 {
 	int ret;
 
+	if (!patch || !patch->mod)
+		return -EINVAL;
+
 	if (!is_livepatch_module(patch->mod)) {
 		pr_err("module %s is not marked as a livepatch module",
 		       patch->mod->name);
@@ -885,9 +888,6 @@ int klp_register_patch(struct klp_patch *patch)
 	if (!klp_initialized())
 		return -ENODEV;
 
-	if (!patch || !patch->mod)
-		return -EINVAL;
-
 	/*
 	 * A reference is taken on the patch module to prevent it from being
 	 * unloaded.  Right now, we don't allow patch modules to unload since