virnetdev: Resolve Coverity FORWARD_NULL

The complaint is that if cleanup is called when virFileReadAll fails,
then mcast->entries is NULL and could be dereferenced in the clear
function. After following the code some - I saw that the caller to
the function (virNetDevGetMulticastTable) will also call
virNetDevMcastListClear if this function returns -1, so this
isn't necessary, so I removed the call.

src/util/virnetdev.c

@@ -2200,9 +2200,6 @@ static int virNetDevGetMcastList(const char *ifname,
 
     ret = 0;
  cleanup:
-    if (ret < 0)
-        virNetDevMcastListClear(mcast);
-
     VIR_FREE(entry);
 
     return ret;