kern/partition: Check for NULL before dereferencing input string

kern/partition: Check for NULL before dereferencing input string

There is the possibility that the value of str comes from an external
source and continuing to use it before ever checking its validity is
wrong. So, needs fixing.

Additionally, drop unneeded part initialization.

Fixes: CID 292444
Signed-off-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Patch-Name: 2021-02-security/029-kern-partition-Check-for-NULL-before-dereferencing-input-string.patch