fixed free(ip) crash

sizeof(*ipstr) would always return 4 on a 32-bit system and 8 on a 64-bit system. (i.e. the size of a char pointer?)
This caused the free(ip); on line 596 to crash.

I'm just a day into c++ and pointers.
- I'm wondering why/how 'ip' was still able to store the entire ip.
- If it stored the 'ip' correctly, why did free() fail? memory corruption during malloc or strcpy?

Btw thanks for the comment on 
http://www.techresx.com/programming/packet-capture-nodejs-edgejs/
Merge pull request #8 from pratham2003/patch-1

fixed free(ip) crash