StringPrep::prepare: reset `error` before calling usprep_prepare().

If the first call to usprep_prepare() returns U_BUFFER_OVERFLOW_ERROR
and the error is not reset, this results in an out-of-memory condition
as all subsequent calls to usprep_prepare() fail without ever setting
`error` to something else.
Merge pull request #35 from directi/buffer-overflow-oom-fix

StringPrep::prepare: reset `error` before calling usprep_prepare().