Fix signed/unsigned comparison warnings

[DanRStevens]

Tagging #11

Fixes the following GCC warnings:

micropather.cpp: In member function ‘void micropather::PathCache::AddItem(const micropather::PathCache::Item&)’:
micropather.cpp:808:14: warning: comparison of integer expressions of different signedness: ‘unsigned int’ and ‘int’ [-Wsign-compare]
   if ( index == allocated )
        ~~~~~~^~~~~~~~~~~~
micropather.cpp: In member function ‘const micropather::PathCache::Item* micropather::PathCache::Find(void*, void*)’:
micropather.cpp:827:14: warning: comparison of integer expressions of different signedness: ‘unsigned int’ and ‘int’ [-Wsign-compare]
   if ( index == allocated )
        ~~~~~~^~~~~~~~~~~~

[DanRStevens]

So this is probably the wrong solution. A size_t value would be much better here, though may require a few more changes.

I don't see much activity on this project, so not sure if there is any interest in fixing this. If so, I'd be happy to rework and submit a more proper fix.