s4/torture: fix compilation in smb2/multichannel

s4/torture: fix compilation in smb2/multichannel

GCC 9.3.0 doesn't like a true array being compared to NULL.

[3628/3972] Compiling source4/torture/smb2/multichannel.c
../../source4/torture/smb2/multichannel.c:1077:7: error: comparison of array 'trees2' equal to a null pointer is always false [-Werror,-Wtautological-pointer-compare]
                if (trees2 == NULL || trees2[i] == NULL) {
                    ^~~~~~    ~~~~
../../source4/torture/smb2/multichannel.c:1284:7: error: comparison of array 'trees2' equal to a null pointer is always false [-Werror,-Wtautological-pointer-compare]
                if (trees2 == NULL || trees2[i] == NULL) {
                    ^~~~~~    ~~~~
../../source4/torture/smb2/multichannel.c:2337:7: error: comparison of array 'trees2' equal to a null pointer is always false [-Werror,-Wtautological-pointer-compare]
                if (trees2 == NULL || trees2[i] == NULL) {
                    ^~~~~~    ~~~~
3 errors generated.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Fri Jul 17 07:16:31 UTC 2020 on sn-devel-184