We're reasonably sure that appending the NUL is not an OOB write, since
the memory stream implementation uses `zend_string` APIs instead of
fiddling with the buffer.
We don't add a regression test because that would require to set up
something in the zend_test extension, and regressions are supposed
to be caught by external consumers of this API, such as mailparse.
Closes GH-15648.