slimbus: Fix multiple corner cases

Initialize the variable before accessing it. Also ensure that it does
not get used outside its intended scope.

Always check for the validity of a pointer before accessing it.

CRs-Fixed: 2121178
Change-Id: I08fdb1bf8445ac9d321eb3433c6a935f34bae437
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>