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/binding-util.cpp

@@ -189,7 +189,7 @@ rb_get_args(int argc, VALUE *argv, const char *format, ...)
 			if (!RB_TYPE_P(tmp, RUBY_T_STRING))
 				rb_raise(rb_eTypeError, "Argument %d: Expected string", argI);
 
-			*s = RSTRING_PTR(tmp);
+			*s = StringValueCStr(tmp);
 			++argI;
 
 			break;

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)