Fix memory leak in virNWFilterDefParseXML()

Fix memory leak in virNWFilterDefParseXML()

While running nwfilterxml2xmltest, it was found that valgrind pointed out the
following error...

==7466== 16 bytes in 1 blocks are definitely lost in loss record 26 of 90
==7466==    at 0x4A06B6F: calloc (vg_replace_malloc.c:593)
    ==7466==    by 0x4C651AD: virAlloc (viralloc.c:142)
    ==7466==    by 0x4D0450D: virNWFilterDefParseNode (nwfilter_conf.c:2575)
    ==7466==    by 0x4D05D84: virNWFilterDefParse (nwfilter_conf.c:2647)
    ==7466==    by 0x401FDE: testCompareXMLToXMLHelper (nwfilterxml2xmltest.c:39)
    ==7466==    by 0x402DE1: virtTestRun (testutils.c:138)
    ==7466==    by 0x4018E9: mymain (nwfilterxml2xmltest.c:111)
    ==7466==    by 0x403482: virtTestMain (testutils.c:593)
    ==7466==    by 0x341F421A04: (below main) (libc-start.c:225)

...21 times, which are related to 21 tests in nwfilterxml2xmltest.c which sent
EXPECT_WARN = false. There were two scenarios in virNWFilterDefParseXML(),
when the variable 'entry' was malloc'ed, but not freed.