wininet: Fix URLCache_LocalFileNameToPathA to return a full path, rather than just the container path.

This was caused by path_len including the nul-terminator and so the rest 
of the string was being added after the nul-terminator, which is 
incorrect. This is fixed by making path_len not include the nul-terminator.

Also fix a few other issues with the function, like not passing a 
correct length into the second call to WideCharToMultiByte, nRequired 
being calculated incorrectly and the string not always being nul-terminated.

Add a test for this function by testing the lpszLocalFileName field 
obtained from RetrieveUrlCacheEntryFileA.