Fix #77821: Potential heap corruption in TSendMail()

Fix #77821: Potential heap corruption in TSendMail()

`zend_string_tolower()` returns a copy (not a duplicate) of the given
string, if it is already in lower case.  In this case we must not not
`zend_string_free()` both strings.  The cleanest solution is to call
` zend_string_release()` on both strings, which properly handles the
refcount.