Clean a bunch of fuzzed files not to fuzz the container.

Clean a bunch of fuzzed files not to fuzz the container.

For some reason pcap fuzzers just *love* to fuzz the "link-layer type
plus other stuff" part of the pcap file header.  That can produce a file
that has non-zero values in reserved fields, which means it can get
rejected well before being handed to anything for dissection.

Clear out one of the reserved parts of that field, as we'll be changing
libpcap to check for non-zero values and reporting an error.