libnetutil's PacketParser uses improper downcasts

[dmiller-nmap]

UBSan complains:

PacketParser.cc:1192:30: runtime error: downcast of address 0x60400000a990 which does not point to an object of type 'ICMPHeader'
0x60400000a990: note: object is of type 'UDPHeader'
 05 00 80 30  90 76 c3 01 00 00 00 00  08 00 00 00 be be be be  50 a9 00 00 40 60 00 00  40 bc 00 00
              ^~~~~~~~~~~~~~~~~~~~~~~
              vptr for 'UDPHeader'
PacketParser.cc:1197:24: runtime error: downcast of address 0x60400000a950 which does not point to an object of type 'ICMPHeader'
0x60400000a950: note: object is of type 'RawData'
 06 00 80 32  f0 80 c3 01 00 00 00 00  2c 01 00 00 be be be be  00 00 00 00 00 00 00 00  90 a9 00 00
              ^~~~~~~~~~~~~~~~~~~~~~~
              vptr for 'RawData'

Due to this code:

      /* So far we've verified that the ICMP error contains an IP datagram that matches
       * what we sent. Now, let's find the upper layer ICMP header (skip extension
       * headers until we find ICMP) */
      ICMPHeader *inner_icmp=(ICMPHeader *)iperror->getNextElement();
      while(inner_icmp!=NULL){
        if(inner_icmp->protocol_id()==HEADER_TYPE_ICMPv4 || inner_icmp->protocol_id()==HEADER_TYPE_ICMPv6 ){
            break;
        }else{
            inner_icmp=(ICMPHeader *)inner_icmp->getNextElement();
        }   
      } 

I don't think protocol_id() is implemented in the base class, which I believe to be PacketElement. We'd need to be sure that the appropriate version of the function gets called for each element (extension header).

Not labeling this a bug because it hasn't caused any visible problems so far.