Merge pull request #27 from kcwu/fix-strgrow

Fix potential heap buffer corruption due to Strgrow
Strcat(Str x, Str y)
{
    STR_LENGTH_CHECK(y);
    Strcat_charp_n(x, y->ptr, y->length);
}

Strcat(Str x, Str y)
{
    STR_LENGTH_CHECK(y);
    Strcat_charp_n(x, y->ptr, y->length);
}
