Ruby

Fix Exception#detailed_message for GC compaction

Before this commit, the test fails with RGENGC_CHECK_MODE enabled:

TestException#test_detailed_message_under_gc_compact_stress [test/ruby/test_exception.rb:1466]:
<"\e[1mfoo (\e[1;4mRuntimeError\e[m\e[1m)\e[m\n" +
"\e[1mbar\e[m\n" +
"\e[1mbaz\e[m"> expected but was
<"\e[1mfoo (\e[1;4mRuntimeError\e[m\e[1m)\e[m\n" +
"\e[1m\x00\x00\x00\x00\x00\x00\x00\e[m">.