scsi: fix buffer overflow in scsi_req_parse_cdb (CVE-2015-5158)

This is a guest-triggerable buffer overflow present in QEMU 2.2.0
and newer.  scsi_cdb_length returns -1 as an error value, but the
caller does not check it.

Luckily, the massive overflow means that QEMU will just SIGSEGV,
making the impact much smaller.

Reported-by: Zhu Donghai (朱东海) <donghai.zdh@alibaba-inc.com>
Fixes: 1894df02811f6b79ea3ffbf1084599d96f316173
Reviewed-by: Fam Zheng <famz@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
