`php_stream_write()` returns `-1` on failure now, which has to be
catered to by `SplFileObject::fputcsv()` which still is supposed to
return `false` on failure.

* PHP-7.3:
  Fixed bug #78910
  Fix #78878: Buffer underflow in bc_shift_addsub
  Fix test
  Fix #78862: link() silently truncates after a null byte on Windows
  Fix #78863: DirectoryIterator class silently truncates after a null byte
  Fix #78943: mail() may release string with refcount==1 twice

* PHP-7.2:
  Fixed bug #78910
  Fix #78878: Buffer underflow in bc_shift_addsub
  Fix test
  Fix #78862: link() silently truncates after a null byte on Windows
  Fix #78863: DirectoryIterator class silently truncates after a null byte

Since the constructor of DirectoryIterator and friends is supposed to
accepts paths (i.e. strings without NUL bytes), we must not accept
arbitrary strings.

If count is ZEND_LONG_MIN the count-- loop underflows. This is
ultimately harmless, but results in a ubsan warning.

Fix this by adding a sanity check that the count isn't negative,
because that doesn't make sense...