qemudDomainAttachSCSIDisk: avoid FP NULL-ptr-deref from clang

* src/qemu/qemu_driver.c (qemudDomainAttachSCSIDisk):
Initialize "cont" to NULL, so clang knows it's set.
Add an sa_assert so it knows it's non-NULL when dereferenced.

src/qemu/qemu_driver.c

@@ -6851,7 +6851,7 @@ static int qemudDomainAttachSCSIDisk(struct qemud_driver *driver,
 {
     int i;
     qemuDomainObjPrivatePtr priv = vm->privateData;
-    virDomainControllerDefPtr cont;
+    virDomainControllerDefPtr cont = NULL;
     char *drivestr = NULL;
     char *devstr = NULL;
     int ret = -1;
@@ -6894,6 +6894,11 @@ static int qemudDomainAttachSCSIDisk(struct qemud_driver *driver,
             goto error;
     }
 
+    /* Tell clang that "cont" is non-NULL.
+       This is because disk->info.addr.driver.controller is unsigned,
+       and hence the above loop must iterate at least once.  */
+    sa_assert (cont);
+
     if (cont->info.type != VIR_DOMAIN_DEVICE_ADDRESS_TYPE_PCI) {
         qemuReportError(VIR_ERR_INTERNAL_ERROR,
                         _("SCSI controller %d was missing its PCI address"), cont->idx);