check Ruby strings for embedded null bytes

The RGSS doesn't do it. But doing it shouldn't be a problem, as it's the
correct way. If a game is broken by this commit (unlikely), then the game
needs to be fixed as silent truncation is highly unlikely to be wanted.

binding-mri/bitmap-binding.cpp

@@ -32,7 +32,7 @@ DEF_TYPE(Bitmap);
 static const char *objAsStringPtr(VALUE obj)
 {
 	VALUE str = rb_obj_as_string(obj);
-	return RSTRING_PTR(str);
+	return StringValueCStr(str);
 }
 
 void bitmapInitProps(Bitmap *b, VALUE self)