Follow #433 #434 to avoid SystemStackError

[azumakuniyuki]

• Fix NoMethodError #433
• fix: guard nil lower chunk in RFC2045.haircut (#433) #434

% ruby -Ilib -rsisimai -e 'Sisimai.rise($*.shift)' ./set-of-emails/should-not-crash/p5-664-iomart-mail-filter.eml
rb-sisimai/lib/sisimai/rfc2045.rb:112:in `parameter': stack level too deep (SystemStackError)
	rb-sisimai/lib/sisimai/rfc2045.rb:129:in `boundary'
	rb-sisimai/lib/sisimai/rfc2045.rb:223:in `levelout'
	rb-sisimai/lib/sisimai/rfc2045.rb:241:in `levelout'
	rb-sisimai/lib/sisimai/rfc2045.rb:241:in `levelout'
	rb-sisimai/lib/sisimai/rfc2045.rb:241:in `levelout'
	rb-sisimai/lib/sisimai/rfc2045.rb:241:in `levelout'
	rb-sisimai/lib/sisimai/rfc2045.rb:241:in `levelout'
	rb-sisimai/lib/sisimai/rfc2045.rb:241:in `levelout'
	 ... 5685 levels...
	rb-sisimai/lib/sisimai/message.rb:82:in `rise'
	rb-sisimai/lib/sisimai/fact.rb:115:in `rise'
	rb-sisimai/lib/sisimai.rb:34:in `rise'
	from -e:1:in `<main>'

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.46%. Comparing base (1369c6e) to head (e3e751b).

Additional details and impacted files

@@             Coverage Diff              @@
##           5-stable     #436      +/-   ##
============================================
- Coverage     95.46%   95.46%   -0.01%     
============================================
  Files           126      126              
  Lines          5401     5400       -1     
============================================
- Hits           5156     5155       -1     
  Misses          245      245              

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