lib-sasl: sasl-server-mech-otp - Fix parsing of authid from initial client response

lib-sasl: sasl-server-mech-otp - Fix parsing of authid from initial client response

Before, it assumed this part to be nul-terminated, while it is not. Therefore,
it could read out-of-bounds memory, possibly crashing, which is wat became
apparent in the fuzzer.

The actual auth service code implicitly always provides a NUL termination
though, because the client response data comes via t_buffer_create() content
which always reserves +1 byte for NUL. Therefore, the auth service would not
have encountered this issue.

OSS-Fuzz report: 456423266