Initialize saveDir and levelDir in FileSystemUtils.cpp

This is just in case these values happen to be used without being
initialized or anything. I vaguely recall someone reporting an issue
where they didn't have a "Documents" folder on Windows and their level
folder ended up being a garbage path, so it's good to do this.

desktop_version/src/FileSystemUtils.cpp

@@ -34,8 +34,8 @@
 #define MAX_PATH PATH_MAX
 #endif
 
-char saveDir[MAX_PATH];
-char levelDir[MAX_PATH];
+char saveDir[MAX_PATH] = {'\0'};
+char levelDir[MAX_PATH] = {'\0'};
 
 void PLATFORM_getOSDirectory(char* output);
 void PLATFORM_migrateSaveData(char* output);