Fix null dereference caused by osFixFileName, remove stray semicolon (#467)

src/platform/nix/main.cpp

@@ -323,6 +323,7 @@ void joyUpdate() {
 // filesystem
 #define MAX_FILES 4096
 char* gFiles[MAX_FILES];
+const char* gEmpty = "";
 int32 gFilesCount;
 
 void addDir(char* path)
@@ -386,7 +387,7 @@ const char* osFixFileName(const char* fileName)
             return gFiles[i];
         }
     }
-    return NULL;
+    return gEmpty;
 }
 
 // system

src/utils.h

@@ -1683,7 +1683,7 @@ extern void osWriteSlot  (Stream *stream);
 
 #ifdef _OS_LINUX
 extern const char* osFixFileName(const char* fileName);
-#endif;
+#endif
 
 #ifdef _OS_WEB
 extern void osDownload   (Stream *stream);