lib: Avoid declaring zero-length VLAs in various messaging functions

lib: Avoid declaring zero-length VLAs in various messaging functions

In messaging_rec_create(), messaging_recv_cb() and
messaging_dispatch_rec(), variable length arrays of file descriptors are
declared using an incoming num_fds parameter.

However, there are several scenarios where num_fds can be zero, and
declaring a zero-length VLA is undefined behavior. This can lead to
segmentation faults and/or other crashes when compiling with recent
versions of clang at high optimization levels.

To avoid ever using zero as the length for these declarations, use
MAX(1, length) instead.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14605

Signed-off-by: Dimitry Andric <dimitry@andric.com>
Reviewed-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Mon Jan  4 10:50:07 UTC 2021 on sn-devel-184

(cherry picked from commit 3e96c95d41e4ccd0bf43b3ee78af644e2bc32e30)