Fix -Wstringop-overflow warnings in fs/smb/client/cifssmb.c

[GustavoARSilva]

Found in powerpc64 and s390 (and a couple more archs).

fs/smb/client/cifssmb.c:2987:31: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
fs/smb/client/cifssmb.c:2988:30: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]

[GustavoARSilva]

Patch: https://lore.kernel.org/linux-hardening/ZK3h3+dHBGONHt+S@work/