get classname and concatenate ": " only once

src/error.c

@@ -149,14 +149,14 @@ exc_inspect(mrb_state *mrb, mrb_value exc)
     }
   }
   else {
-    str = mrb_str_new_cstr(mrb, mrb_obj_classname(mrb, exc));
+    const char *cname = mrb_obj_classname(mrb, exc);
+    str = mrb_str_new_cstr(mrb, cname);
+    mrb_str_cat_lit(mrb, str, ": ");
     if (append_mesg) {
-      mrb_str_cat_lit(mrb, str, ": ");
       mrb_str_append(mrb, str, mesg);
     }
     else {
-      mrb_str_cat_lit(mrb, str, ": ");
-      mrb_str_cat_cstr(mrb, str, mrb_obj_classname(mrb, exc));
+      mrb_str_cat_cstr(mrb, str, cname);
     }
   }
   return str;