Revert "megasas: remove useless check for cmd->frame"

This reverts commit 8cc46787b5b58f01a11c919c7ff939ed009e27fc.
It turns out that cmd->frame can be NULL and thus the commit
can cause a SIGSEGV

Reported-by: Holger Schranz <holger@fam-schranz.de>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
