Fixing several buffer overflow in parse_arguments in n64split

[Sn0wfreezeDev]

Hi,

for a university project we had look for buffer overflows in open source projects.
We have tested your software and discovered several buffer overflows in the n64split tool.

We have fixed those in this commit using strncpy instead of strcpy.

To reproduce the buffer overflow compile n64split with the gcc flag -fno-stack-protector and run it with the following input:
./n64split $(python -c "print ''.join(c * 4096 for c in 'ABC') + 'D' * 72")

This will create a segmentation fault, because it will overwrite among others the return address of the main function.