[Dovecot] auth crash in 2.0.3

[Mike Abbott]

Here's another one-off crash in dovecot-2.0.3's auth process.  Looks like request->mech == NULL in auth_request_initial().  I don't see any obvious fixes to this code between 2.0.3 and 2.0.6 so it may still be present in 2.0.6.

Exception Type:  EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x0000000000000018
0   auth                                0x000000010d89381b auth_request_initial + 85
1   auth                                0x000000010d891844 auth_penalty_anvil_callback + 207
2   libdovecot.0.dylib                  0x000000010d8ea44f anvil_client_connect + 364
3   libdovecot.0.dylib                  0x000000010d8f77e9 io_loop_handler_run + 234
4   libdovecot.0.dylib                  0x000000010d8f82bb io_loop_run + 56
5   libdovecot.0.dylib                  0x000000010d8edf98 master_service_run + 27
6   auth                                0x000000010d89b18a main + 674
7   auth                                0x000000010d88ec14 start + 52

There were several "Timeout waiting for handshake from auth server" errors from imap-login and pop3-login in the minutes before this crash.