json_patch: treat an empty "from" as a parent of any "path"

Since the token-boundary check for the parent/child test (1b504e2), an
empty "from" (the root) only counts as a prefix of "path" when "path"
starts with '/'. With an invalid "path" like "a", a move from "" went
on to remove the root: __json_patch_apply_remove() dropped a reference
to the document, the following set failed on the bad path, and the
error path dropped the last reference, so *base was freed while the
caller still owned it (use-after-free / double free in
json_object_put()).

The root is a parent of every other location, so an empty "from" now
always takes the parent/child branch: a no-op for "path": "" and
EINVAL otherwise. This also restores the invariant the comment below
relies on, that json_pointer can't find the root here.

Added move and copy cases to json_patch_tests.json and regenerated
test_json_patch.expected.

json_patch.c

@@ -292,7 +292,7 @@ static int json_patch_apply_move_copy(struct json_object **res,
 	 * sibling keys that happen to share a name prefix.
 	 */
 	if (strncmp(from_s, path, from_s_len) == 0 &&
-	    (path[from_s_len] == '\0' || path[from_s_len] == '/')) {
+	    (from_s_len == 0 || path[from_s_len] == '\0' || path[from_s_len] == '/')) {
 		/**
 		 * If lengths match, it's a noop, if they don't,
 		 * then we're trying to move a parent under a child