Fix ObjectSpace.dump with super() callinfo

super() uses 0 as mid for its callinfo, so we need to check for that to
avoid a segfault when using dump_all.

ext/objspace/objspace_dump.c

@@ -379,6 +379,7 @@ dump_object(VALUE obj, struct dump_config *dc)
     rb_io_t *fptr;
     ID flags[RB_OBJ_GC_FLAGS_MAX];
     size_t n, i;
+    ID mid;
 
     if (SPECIAL_CONST_P(obj)) {
         dump_append_special_const(dc, obj);
@@ -433,9 +434,12 @@ dump_object(VALUE obj, struct dump_config *dc)
 
         switch (imemo_type(obj)) {
           case imemo_callinfo:
-            dump_append(dc, ", \"mid\":\"");
+            mid = vm_ci_mid((const struct rb_callinfo *)obj);
-            dump_append(dc, RSTRING_PTR(rb_id2str(vm_ci_mid((const struct rb_callinfo *)obj))));
+            if (mid != 0) {
-            dump_append(dc, "\"");
+                dump_append(dc, ", \"mid\":\"");
+                dump_append(dc, rb_id2name(mid));
+                dump_append(dc, "\"");
+            }
             break;
 
           default:

test/objspace/test_objspace.rb

@@ -562,6 +562,27 @@ class TestObjSpace < Test::Unit::TestCase
     end
   end
 
+  def test_dump_callinfo_includes_mid
+    assert_in_out_err(%w[-robjspace], "#{<<-"begin;"}\n#{<<-'end;'}") do |output, error|
+      begin;
+        class Foo
+          def foo
+            super(bar: 123) # should not crash on 0 mid
+          end
+
+          def bar
+            baz(bar: 123) # mid: baz
+          end
+        end
+
+        ObjectSpace.dump_all(output: $stdout)
+      end;
+      assert_empty error
+      assert(output.count > 1)
+      assert_equal 1, output.count { |l| l.include?('"mid":"baz"') }
+    end
+  end
+
   def test_dump_string_coderange
     assert_includes ObjectSpace.dump("TEST STRING"), '"coderange":"7bit"'
     unknown = "TEST STRING".dup.force_encoding(Encoding::BINARY)