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.

tests/json_patch_tests.json

@@ -480,6 +480,16 @@
       "patch": [{"op": "copy", "from": "/aa", "path": "/aab"}],
       "expected": {"aa": 1, "aab": 1} },
 
+    { "comment": "Moving the root to a path without a leading '/' must fail, not free the document",
+      "doc": {"a": null},
+      "patch": [{"op": "move", "from": "", "path": "a"}],
+      "error": "the root is a parent of every other location" },
+
+    { "comment": "Copying the root to a path without a leading '/' must fail",
+      "doc": {"a": null},
+      "patch": [{"op": "copy", "from": "", "path": "a"}],
+      "error": "the root is a parent of every other location" },
+
     { "comment": "Remove members whose names need escaping",
       "doc": {"a/b": 1, "m~n": 2, "c": 3},
       "patch": [{"op": "remove", "path": "/a~1b"},