[libFuzzer] Properly use unsigned for Process ID.

Use unsigned for PID instead of signed int. GetCurrentProcessId() returns
an unsigned (DWORD) so we must be sure we can deal with all possible values.
I use a long unsigned to be sure it can hold a 32 bit unsigned (DWORD).

Differential Revision: https://reviews.llvm.org/D27281

llvm-svn: 289558