Fix infinite loop at illegal sequence [Bug #17729]

	As mblen returns -1 on failure, skip the first byte and try the
	succeeding bytes in that case.

	Close https://github.com/ruby/ruby/pull/4281
	---
	 eval_intern.h                 | 11 ++++++++++-
	 test/ruby/test_rubyoptions.rb |  5 +++++
	 2 files changed, 15 insertions(+), 1 deletion(-)