virsh: fix return value in storage vol name completor

The function must return a pointer, not a boolean. Fortunately 'false'
is equivalent to 'NULL' so this bug no had ill effect previously.
Reviewed-by: NJán Tomko <jtomko@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

tools/virsh-completer.c

@@ -287,7 +287,7 @@ virshStorageVolNameCompleter(vshControl *ctl,
         return NULL;
 
     if (!(pool = virshCommandOptPool(ctl, cmd, "pool", NULL)))
-        return false;
+        return NULL;
 
     if ((nvols = virStoragePoolListAllVolumes(pool, &vols, flags)) < 0)
         goto error;