Fix make check

cc -DHAVE_CONFIG_H -I. -I..  -I.. -I../tests    -O2 -pipe  -fstack-protector-strong -fno-strict-aliasing  -Wall -Werror -Wcast-qual -Wno-error=deprecated-declarations -Wextra -Wno-unused-parameter -D_GNU_SOURCE -D_REENTRANT -MT test_parse.o -MD -MP -MF .deps/test_parse.Tpo -c -o test_parse.o test_parse.c
test_parse.c:256:14: error: adding 'int' to a string does not append to the string [-Werror,-Wstring-plus-int]
        { "null123" + 4,       4, 3, json_tokener_success, 1 },
          ~~~~~~~~~~^~~
test_parse.c:256:14: note: use array indexing to silence this warning
        { "null123" + 4,       4, 3, json_tokener_success, 1 },
                    ^
          &         [  ]
test_parse.c:258:12: error: adding 'int' to a string does not append to the string [-Werror,-Wstring-plus-int]
        { "nullx" + 4,         2, 0, json_tokener_error_parse_unexpected, 1 },
          ~~~~~~~~^~~
test_parse.c:258:12: note: use array indexing to silence this warning
        { "nullx" + 4,         2, 0, json_tokener_error_parse_unexpected, 1 },
                  ^
          &       [  ]
test_parse.c:260:25: error: adding 'int' to a string does not append to the string [-Werror,-Wstring-plus-int]
        { "{\"a\":1}{\"b\":2}" + 7,
          ~~~~~~~~~~~~~~~~~~~~~^~~
test_parse.c:260:25: note: use array indexing to silence this warning
        { "{\"a\":1}{\"b\":2}" + 7,
                               ^
          &                    [  ]
3 errors generated.
*** Error code 1

Stop.

tests/test_parse.c

@@ -303,11 +303,11 @@ struct incremental_step {
 	   the next few tests check that parsing multiple sequential
        json objects in the input works as expected */
 	{ "null123",           9, 4, json_tokener_success, 0 },
-	{ "null123" + 4,       4, 3, json_tokener_success, 1 },
+	{ &"null123"[4],       4, 3, json_tokener_success, 1 },
 	{ "nullx",             5, 4, json_tokener_success, 0 },
-	{ "nullx" + 4,         2, 0, json_tokener_error_parse_unexpected, 1 },
+	{ &"nullx"[4],         2, 0, json_tokener_error_parse_unexpected, 1 },
 	{ "{\"a\":1}{\"b\":2}",15, 7, json_tokener_success, 0 },
-	{ "{\"a\":1}{\"b\":2}" + 7,
+	{ &"{\"a\":1}{\"b\":2}"[7],
 	                       8, 7, json_tokener_success, 1 },
 
 	/* Some bad formatting. Check we get the correct error status */