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.

json_pointer.c

@@ -102,16 +102,10 @@ static int json_pointer_get_single_path(struct json_object *obj, char *path,
 			return -1;
 		}
 
-		obj = json_object_array_get_idx(obj, *idx);
-		if (obj)
-		{
-			if (value)
-				*value = obj;
-			return 0;
-		}
-		/* Entry not found */
-		errno = ENOENT;
-		return -1;
+		/* A null element is stored as NULL, but it exists */
+		if (value)
+			*value = json_object_array_get_idx(obj, *idx);
+		return 0;
 	}
 
 	/* RFC states that we first must eval all ~1 then all ~0 */