This patch addresses the following coverity findings:
/libvirt/src/conf/nwfilter_params.c:390:
var_assigned: Assigning: "varValue" = null return value from "virHashLookup".
/libvirt/src/conf/nwfilter_params.c:392:
dereference: Dereferencing a pointer that might be null "varValue" when calling "virNWFilterVarValueGetNthValue".
/libvirt/src/conf/nwfilter_params.c:399:
dereference: Dereferencing a pointer that might be null "tmp" when calling "virNWFilterVarValueGetNthValue".