json_pointer: find null array elements

json_pointer_get_single_path() returned ENOENT when the array element
at an in-range index was NULL, which is how json-c stores a JSON null.
A null object member was already found, but on [0, null, 2] "/1" failed
with "not found", although RFC 6901 says it references the null value.

The array index is bounds-checked just before, so a NULL element now
resolves like a null member: json_pointer_get() returns 0 and sets
*res to NULL. This also fixes json_patch_apply(), which failed every
"test", "replace" and "remove" on a null array element, and every
"move" and "copy" from one, with "Did not find element referenced by
path field".

Added a json_pointer_get() test, and json_patch_tests.json cases that
mirror the existing null object member ones.

tests/test_json_pointer.expected

@@ -12,6 +12,7 @@ PASSED - GET -  /i\j == 5
 PASSED - GET -  /k"l == 6
 PASSED - GET -  /  == 7
 PASSED - GET -  /m~0n == 8
+PASSED - GET - /1 == null
 { "arr": [ { "obj": [ { }, { }, { "obj1": 0, "obj2": "1" } ] } ], "obj": { "obj": { "obj": [ { "obj1": 0, "obj2": "1" } ] } } }
 PASSED - GET - RECURSION TEST
 PASSED - GET - LOADED TEST JSON