Don't treat the packet length as unsigned.

The scanf family of functions are as annoyingly bad at handling unsigned
numbers as strtoul() is - both of them are perfectly willing to accept a
value beginning with a negative sign as an unsigned value.  When using
strtoul(), you can compensate for this by explicitly checking for a '-'
as the first character of the string, but you can't do that with
sscanf().

So revert to having pkt_len be signed, and scanning it with %d, but
check for a negative value and fail if we see a negative value.

Bug: 12395
Change-Id: I43b458a73b0934e9a5c2c89d34eac5a8f21a7455
Reviewed-on: https://code.wireshark.org/review/15223
Reviewed-by: Guy Harris <guy@alum.mit.edu>