ntlm_wb: Fix theoretical memory leak

Static analysis indicated that my commit 9008f3d5 ("ntlm_wb: Fix
hard-coded limit on NTLM auth packet size") introduced a potential
memory leak on an error path, because we forget to free the buffer
before returning an error.

Fix this.

Although actually, it never happens in practice because we never *get*
here with state == NTLMSTATE_TYPE1. The state is always zero. That
might want cleaning up in a separate patch.

Reported-by: Terri Oda