virsh-nodedev: Resolve Coverity issues

Recent changes uncovered FORWARD_NULL and NEGATIVE_RETURNS problems with
the processing of the 'ndevices' and its associated allocated arrays in
'vshNodeDeviceListCollect' due to the possibility of returning -1 in a
call and using the returned value as a for loop index end condition.

tools/virsh-nodedev.c

@@ -352,7 +352,7 @@ finished:
     success = true;
 
 cleanup:
-    for (i = 0; i < ndevices; i++)
+    for (i = 0; ndevices != -1 && i < ndevices; i++)
         VIR_FREE(names[i]);
     VIR_FREE(names);