#2 more thorough packet checking

[simon lundell]

This patch adds a more thorough checking of the packet headers. Without this patch, invalid packets were treated as valid and cased segfaults later on. It also fixes a minor bug where to little memory is reserved for the output directory string.

[simon lundell]

output-dir-str-fix+invalid-packet-detect

[stvlange]

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Getting this on ubuntu when I try to apply patch, any thoughts are appreciated.

Hunk #4 FAILED at 204.
Hunk #5 FAILED at 222.
Hunk #6 FAILED at 248.
Hunk #7 FAILED at 294.
4 out of 8 hunks FAILED -- saving rejects to file tcpxtract.rej

tcpxtract.rej:

***************
*** 194,195
- if (payload_size <= 0)
- return;
--- 204 -----
+
***************
*** 213
-
--- 0 -----
***************
*** 240
- char *dev; /* Sniffing devise */
--- 248 -----
+ char *dev = NULL; /* Sniffing device */
***************
*** 286
- output_prefix = emalloc(strlen(optarg) + 1);
--- 294 -----
+ output_prefix = emalloc(strlen(optarg) + 2);