conf: fix a memory leak when parsing nat port XML nodes

conf: fix a memory leak when parsing nat port XML nodes

 ==5306== 8 bytes in 1 blocks are definitely lost in loss record 24 of 277
 ==5306==    at 0x4C28B2F: calloc (vg_replace_malloc.c:593)
 ==5306==    by 0x5293CAF: virAllocN (viralloc.c:152)
 ==5306==    by 0x52DFEAE: virXPathNodeSet (virxml.c:611)
 ==5306==    by 0x5313DD9: virNetworkDefParseXML (network_conf.c:1408)
 ==5306==    by 0x53170F6: virNetworkObjUpdateParseFile (network_conf.c:2031)
 ==5306==    by 0x131DA63C: networkStartup (bridge_driver.c:279)
 ==5306==    by 0x53481DF: virStateInitialize (libvirt.c:822)
 ==5306==    by 0x40DF44: daemonRunStateInit (libvirtd.c:877)
 ==5306==    by 0x52D2FF5: virThreadHelper (virthreadpthread.c:161)
 ==5306==    by 0x5D00C52: start_thread (in /usr/lib64/libpthread-2.17.so)
 ==5306==    by 0x6410ECC: clone (in /usr/lib64/libc-2.17.so)