Check if the backend option is missing before searching the backend
table. This fixes a NULL pointer dereference when QEMU is invoked with
the following invalid command-line:
$ qemu -chardev id=foo,path=/tmp/socket
Previously QEMU would segfault, now it produces this error message:
chardev: "foo" missing backend
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>