For example, consider the following SecretString construction:
String foo = "foo";
auto ss = SecretString::take_ownership(foo.to_byte_buffer());
The ByteBuffer created by to_byte_buffer() will not contain the NUL
terminator. Therefore, the value returned by SecretString::characters
will not be NUL-terminated either.
Currently, the only use of SecretString is to pass its character data to
crypt(), which requires a NUL-terminated string. To ensure this cannot
result in a buffer overrun, make SecretString append a NUL terminator to
its buffer if there isn't one already.
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