virsh: Don't infloop on snapshot/storage_vol failure

Most of our completers used the pattern:
if ((nITEM = virITEMListAll()) < 0)
    return NULL;

but the virDomainSnapshot and virStorageVolume completers were instead
using goto error. If the ListAll fails with -1, the cleanup label was
running a loop of 'size_t i < int nITEM', which is an extreme waste of
CPU cycles. Broken since their introduction in v4.1.

Fixes: f81f8b62
Fixes: 4cb4b649Reported-by: NJán Tomko <jtomko@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>

tools/virsh-completer.c

@@ -278,6 +278,7 @@ virshStorageVolNameCompleter(vshControl *ctl,
     virshControlPtr priv = ctl->privData;
     virStoragePoolPtr pool = NULL;
     virStorageVolPtr *vols = NULL;
+    int rc;
     int nvols = 0;
     size_t i = 0;
     char **ret = NULL;
@@ -290,8 +291,9 @@ virshStorageVolNameCompleter(vshControl *ctl,
     if (!(pool = virshCommandOptPool(ctl, cmd, "pool", NULL)))
         return NULL;
 
-    if ((nvols = virStoragePoolListAllVolumes(pool, &vols, flags)) < 0)
+    if ((rc = virStoragePoolListAllVolumes(pool, &vols, flags)) < 0)
         goto error;
+    nvols = rc;
 
     if (VIR_ALLOC_N(ret, nvols + 1) < 0)
         goto error;
@@ -631,6 +633,7 @@ virshSnapshotNameCompleter(vshControl *ctl,
     virshControlPtr priv = ctl->privData;
     virDomainPtr dom = NULL;
     virDomainSnapshotPtr *snapshots = NULL;
+    int rc;
     int nsnapshots = 0;
     size_t i = 0;
     char **ret = NULL;
@@ -643,8 +646,9 @@ virshSnapshotNameCompleter(vshControl *ctl,
     if (!(dom = virshCommandOptDomain(ctl, cmd, NULL)))
         return NULL;
 
-    if ((nsnapshots = virDomainListAllSnapshots(dom, &snapshots, flags)) < 0)
+    if ((rc = virDomainListAllSnapshots(dom, &snapshots, flags)) < 0)
         goto error;
+    nsnapshots = rc;
 
     if (VIR_ALLOC_N(ret, nsnapshots + 1) < 0)
         goto error;