virsh: fixing segfault by pool autocompleter function.

The commands which requires a pool to perform any action for a volume is
throwing a segfault when you pass the volume name before a pool name or
without the argument '--pool'.

An example that works:
virsh # vol-list loops-pool
 Name                 Path
-------------------------------------------------------------------
 loop0                /mnt/loop0

virsh # vol-info --pool loops-pool lo<TAB>

An example that does not work:
virsh # vol-list loops-pool
 Name                 Path
-------------------------------------------------------------------
 loop0                /mnt/loop0

virsh # vol-info lo<TAB>
Segmentation Fault

The example 'vol-info' can be executed as 'vol-info loop0 --pool
loops-pool'. So, this commit fixes this problem when the arguments are
inverted and avoids the segfault.
Signed-off-by: NJulio Faracco <jcfaracco@gmail.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

tools/virsh-pool.c

@@ -143,6 +143,9 @@ virshCommandOptPoolBy(vshControl *ctl, const vshCmd *cmd, const char *optname,
     if (vshCommandOptStringReq(ctl, cmd, optname, &n) < 0)
         return NULL;
 
+    if (cmd->skipChecks && !n)
+        return NULL;
+
     vshDebug(ctl, VSH_ERR_INFO, "%s: found option <%s>: %s\n",
              cmd->def->name, optname, n);