Fix bug #15420

smbd crashes through an uninitialized pointer over SMB1 with a truncated sessionsetup request.

https://bugzilla.samba.org/show_bug.cgi?id=15420

Patchset does:

1). Force the pointer to be bad to make the crash repeatable. 2). Add new regression test and knownfail. 3). Fix it and remove knownfail 4). Change forced bad pointer in (1) to NULL initialization. 5). Fix all other callers of the error-prone function for completeness.