fix: convert to different union type crash

[zhongwencool]

1. Define a union type union([ref(log_file_handler),map(name, ref(log_file_handler))]
2. Convert ref(log_file_handler) to map(name, ref(log_file_handler))
3. Crash

**throw:{#{roots =>
       [{f1,#{converter => #Fun<hocon_tconf_tests.57.87224844>,
              type =>
                  {union,[{ref,hocon_tconf_tests,bar},
                          {map,name,{ref,hocon_tconf_tests,bar}}]}}}]},
 [#{kind => validation_error,
    mismatches =>
        #{"bar" =>
              #{kind => validation_error,path => "f1",
                reason => unknown_fields,unknown => "default",
                unmatched => "host,union_with_default"},
          "name" =>
              #{kind => validation_error,
                location => #{line => 1},
                path => "f1.field1",reason => bad_value_for_struct,
                value => <<"foo1">>}},
    path => "f1",reason => matched_no_union_member}]}
  output:<<"">>

More detail see test/hocon_tconf_tests.erl