suppress a couple of spurious warnings in scsi-generic.c

This patch fixes two spurious `may be used uninitialised' warnings
when compiling with some compilers.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5127 c046a42c-6fe2-441c-8c8c-71466251a162
