src: fix warning in cares_wrap.cc

This commit fixes the following warning:

./src/cares_wrap.cc:1268:5: warning: comparison of integers of
    different signs: 'uint32_t' (aka 'unsigned int') and 'int'
    [-Wsign-compare]
    CHECK_EQ(ret->Length(), a_count + aaaa_count);

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines