nwfilter: Fix memory leak

Below patch fixes this coverity report:

/libvirt/src/conf/nwfilter_conf.c:382:
leaked_storage: Variable "varAccess" going out of scope leaks the storage it points to.

(cherry picked from commit b1675bac)

src/conf/nwfilter_conf.c

@@ -375,6 +375,7 @@ virNWFilterRuleDefAddVar(virNWFilterRuleDefPtr nwf,
 
     if (VIR_EXPAND_N(nwf->varAccess, nwf->nVarAccess, 1) < 0) {
         virReportOOMError();
+        virNWFilterVarAccessFree(varAccess);
         return -1;
     }