virsh: plug memory leak on cmdDomIfGetLink() sucessful path

Detected by valgrind. Leak introduced in commit dc675f37:

* tools/virsh.c: fix memory leak on cmdDomIfGetLink.

* how to reproduce?
  % valgrind -v --leak-check=full virsh domif-getlink <domain name> 0

* actual valgrind result:

==13102== 18 bytes in 1 blocks are definitely lost in loss record 9 of 47
==13102==    at 0x4A05FDE: malloc (vg_replace_malloc.c:236)
==13102==    by 0x322A6A67DD: xmlStrndup (in /usr/lib64/libxml2.so.2.7.6)
==13102==    by 0x414892: cmdDomIfGetLink (virsh.c:1538)
==13102==    by 0x4136A2: vshCommandRun (virsh.c:16363)
==13102==    by 0x4253FB: main (virsh.c:17865)
==13102==
==13102== LEAK SUMMARY:
==13102==    definitely lost: 18 bytes in 1 blocks
==13102==    indirectly lost: 0 bytes in 0 blocks
==13102==      possibly lost: 0 bytes in 0 blocks
==13102==    still reachable: 127,888 bytes in 1,361 blocks
==13102==         suppressed: 0 bytes in 0 blocks
Signed-off-by: NAlex Jia <ajia@redhat.com>

tools/virsh.c

@@ -1541,6 +1541,7 @@ cmdDomIfGetLink (vshControl *ctl, const vshCmd *cmd)
                     VIR_FREE(mac);
                     goto hit;
                 }
+                VIR_FREE(mac);
             }
             cur = cur->next;
         }