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/test_json_patch.expected

@@ -131,6 +131,10 @@ Testing '(null)', doc '{ "foo": [ "bar", "baz" ], "": 0, "a\/b": 1, "c%d": 2, "e
 Testing 'Move to same location has no effect', doc '{ "foo": 1 }' patch '[ { "op": "move", "from": "\/foo", "path": "\/foo" } ]' : OK
 Testing 'Move between sibling keys sharing a name prefix is not a parent/child move', doc '{ "aa": 1, "aab": 2 }' patch '[ { "op": "move", "from": "\/aa", "path": "\/aab" } ]' : OK
 Testing 'Copy between sibling keys sharing a name prefix is not a parent/child move', doc '{ "aa": 1, "aab": 2 }' patch '[ { "op": "copy", "from": "\/aa", "path": "\/aab" } ]' : OK
+Testing 'Moving the root to a path without a leading '/' must fail, not free the document', doc '{ "a": null }' patch '[ { "op": "move", "from": "", "path": "a" } ]' : OK
+ => json_patch_apply failed as expected: ERRNO=EINVAL at patch idx 0: Invalid attempt to move parent under a child
+Testing 'Copying the root to a path without a leading '/' must fail', doc '{ "a": null }' patch '[ { "op": "copy", "from": "", "path": "a" } ]' : OK
+ => json_patch_apply failed as expected: ERRNO=EINVAL at patch idx 0: Invalid attempt to move parent under a child
 Testing 'Remove members whose names need escaping', doc '{ "a\/b": 1, "m~n": 2, "c": 3 }' patch '[ { "op": "remove", "path": "\/a~1b" }, { "op": "remove", "path": "\/m~0n" } ]' : OK
 Testing 'Move from a member whose name needs escaping', doc '{ "a\/b": 1 }' patch '[ { "op": "move", "from": "\/a~1b", "path": "\/c" } ]' : OK
 Testing '(null)', doc '{ "foo": 1, "baz": [ { "qux": "hello" } ] }' patch '[ { "op": "move", "from": "\/foo", "path": "\/bar" } ]' : OK