nwfilter: address coverity findings

nwfilter: address coverity findings

This patch addresses the following coverity findings:

/libvirt/src/conf/nwfilter_params.c:157:
deref_parm: Directly dereferencing parameter "val".

/libvirt/src/conf/nwfilter_params.c:473:
negative_returns: Using variable "iterIndex" as an index to array "res->iter".

/libvirt/src/nwfilter/nwfilter_ebiptables_driver.c:2891:
unchecked_value: No check of the return value of "virAsprintf(&protostr, "-d 01:80:c2:00:00:00 ")".

/libvirt/src/nwfilter/nwfilter_ebiptables_driver.c:2894:
unchecked_value: No check of the return value of "virAsprintf(&protostr, "-p 0x%04x ", l3_protocols[protoidx].attr)".

/libvirt/src/nwfilter/nwfilter_ebiptables_driver.c:3590:
var_deref_op: Dereferencing null variable "inst".