Properly check the return value of CCWAddressAsString

It returns NULL on failure. Checking if the negation of it
is less than zero makes no sense. (Found by coverity after moving
the code)

In another case, the return value wasn't checked at all.

(cherry picked from commit 3fe9d75a)

Conflicts:
	src/conf/domain_addr.c - no code movement from commit b2626755

src/qemu/qemu_command.c

@@ -1131,7 +1131,7 @@ int qemuDomainCCWAddressAssign(virDomainDeviceInfoPtr dev,
             goto cleanup;
         }
     } else if (autoassign && !dev->addr.ccw.assigned) {
-        if (!(addr = qemuCCWAddressAsString(&addrs->next)) < 0)
+        if (!(addr = qemuCCWAddressAsString(&addrs->next)))
             goto cleanup;
 
         while (virHashLookup(addrs->defined, addr)) {
@@ -1141,7 +1141,8 @@ int qemuDomainCCWAddressAssign(virDomainDeviceInfoPtr dev,
                 goto cleanup;
             }
             VIR_FREE(addr);
-            addr = qemuCCWAddressAsString(&addrs->next);
+            if (!(addr = qemuCCWAddressAsString(&addrs->next)))
+                goto cleanup;
         }
         dev->addr.ccw = addrs->next;
         dev->addr.ccw.assigned = true;