Merge pull request #27 from kcwu/fix-strgrow

Fix potential heap buffer corruption due to Strgrow
Strfree(Str x)
{
    GC_free(x->ptr);
    GC_free(x);
}

Strfree(Str x)
{
    GC_free(x->ptr);
    GC_free(x);
}
