Rewrite void* arithmetic

cppcheck reported:
[chessbox/chessbox_pgn.c:51]: (portability) 'bufptr' is of type 'void *'. When using void pointers in calculations, the behaviour is undefined.

Change-Id: Ib2139489fa316fb61b4bcdc08f17b97eb6ad0be7

apps/plugins/chessbox/chessbox_pgn.c

@@ -48,7 +48,7 @@ static void *pl_malloc(size_t size)
     }
     else
     {
-        bufptr += size;
+        bufptr = (char*)(bufptr) + size;
         bufleft -= size;
         return ptr;
     }