virsh: Fix regression of vol-resize

Introduced by commit 1daa4ba33a. vshCommandOptStringReq returns
0 on *success* or the option is not required && not present, both
are right result. Error out when returning 0 is not correct.
the caller, it doesn't have to check wether it

tools/virsh-volume.c

@@ -1076,7 +1076,7 @@ cmdVolResize(vshControl *ctl, const vshCmd *cmd)
     if (!(vol = vshCommandOptVol(ctl, cmd, "vol", "pool", NULL)))
         return false;
 
-    if (vshCommandOptStringReq(ctl, cmd, "capacity", &capacityStr) <= 0)
+    if (vshCommandOptStringReq(ctl, cmd, "capacity", &capacityStr) < 0)
         goto cleanup;
     virSkipSpaces(&capacityStr);
     if (*capacityStr == '-') {