[PATCH 1/2] login-common: Fix crash when XCLIENT FORWARD= base64 contains NUL byte

[PATCH 1/2] login-common: Fix crash when XCLIENT FORWARD= base64 contains NUL byte

Sending "XCLIENT FORWARD=AA==" (base64 for a single NUL byte) followed
by a regular login crashed pop3-login (and other login services) with:

  Panic: file ../../src/lib/array.h: line 275 (array_idx_i):
  assertion failed: (idx < array->buffer->used / array->element_size)

p_strsplit_tabescaped() truncates at the first NUL, so an all-NUL
payload produced an empty fields list. forward_fields was still
created (but empty), and the later array_front() call in
sasl_server_auth_begin() then panicked on the empty array.

Reject empty payloads and payloads containing NUL bytes during base64
decode, so the array is never created in this case.

Gbp-Pq: Name 0001-login-common-Fix-crash-when-XCLIENT-FORWARD-base64-c.patch