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 have to properly determine the output length of the conversion to
multi byte strings.

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()`.

Instead of allocating a temporary `OLECHAR` buffer, we allocate the
final `BSTR` right away, and do the multibyte conversion into its
buffer.

We also refactor to guard statements for better readability.