Prevents a potential integer wraparound and heap overflow.

[sgayou]

Attempts to address #54.

Before:

$ ./crash.py 
*** glibc detected *** /usr/bin/python: double free or corruption (!prev): 0x089c9b38 ***
Segmentation fault (core dumped)

After:

$ ./crash.py 
Traceback (most recent call last):
  File "./crash.py", line 3, in <module>
    subprocess._posixsubprocess.fork_exec("a",'a'*0x3FFFFFFF,"a",set(),"a","a",1,1,1,1,1,1,1,3,1,1,"a")
OverflowError

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[gpshead]

While more pedantically technically correct, i'm just going to take the easier approach over in my other PR #55 of capping the number of args to an always within reason number.

[sgayou]

Works for me. You're gonna miss out on support for exabytes worth of arguments though! (kidding)

[gpshead]

I consider that a feature. :)