Allocate zend_strings with correct size

Allocate zend_strings with correct size

For me (32bit) sizeof(zend_string) is 20, which means that the
char[1] array at the end is padded with three bytes. Thus allocating
based on sizeof(zend_string)-1 overallocates by those 3 padding bytes.

This commit fixes the allocation size, by using XtOffsetOf.