rename vncpcap2john.cpp to vncpcap2john.c
turn vncpcap2john.c into C

while staying as near as possible to the original code.
vncpcap2john.c: fix memleaks
vncpcap2john.c: initialize pointers
vncpcap2john.c: remove unneeded check

free(0) is allowed and guarantueed to be a no-op by POSIX.
fix Makefile for vncpcap2john
Merge pull request #533 from rofl0r/noc++

No C++