A `BSTR` is similar to a `zend_string`; it stores the length of the
string just before the actual string, and thus the string may contain
NUL bytes. However, `php_com_olestring_to_string()` is supposed to
deal with arbitrary `OLECHAR*`s which may not be `BSTR`s, so we
introduce `php_com_bstr_to_string()` and use it for the only case where
we actually have to deal with `BSTR`s which may contain NUL bytes.
Contrary to `php_com_olestring_to_string()` we return a `zend_string`,
so we can save the re-allocation when converting to a `zval`.
We also cater to `php_com_string_to_olestring()` not being binary safe,
with basically the same fix we did for `php_com_olestring_to_string()`.
a385cfa7ad