Fix integer overflow within wt2999_join_extractor csuite test

 [2021/06/28 05:05:12.512] ================================

 [2021/06/28 05:05:12.512] ../../../test/csuite/wt2999_join_extractor/main.c:122:13: runtime error: signed integer overflow: 2147450890 + 65536 cannot be represented in type 'int'

 [2021/06/28 05:05:12.512]     #0 0x42329a in main /data/mci/ca8a7d950f1bf956b286e945a18a82e9/wiredtiger/build_posix/test/csuite/../../../test/csuite/wt2999_join_extractor/main.c:122:13

 [2021/06/28 05:05:12.512]     #1 0x7fedf0651bf6 in __libc_start_main /build/glibc-S9d2JN/glibc-2.27/csu/../csu/libc-start.c:310

 [2021/06/28 05:05:12.512]     #2 0x4035d9 in _start (/data/mci/ca8a7d950f1bf956b286e945a18a82e9/wiredtiger/build_posix/test/csuite/test_wt2999_join_extractor+0x4035d9)

 [2021/06/28 05:05:12.512] FAIL test_wt2999_join_extractor (exit status: 1)

    key = 10;

    val[0] = 20;

    val[1] = 30;

    for (i = 0; i < 100000; ++i) {

        key += i;

        val[0] += i;

        val[1] += i;

        cursor1->set_key(cursor1, &k);

        cursor1->set_value(cursor1, &v);

        testutil_check(cursor1->insert(cursor1));

    }