Fix a consistency bug of ISEQ_COVERAGE [Bug #13305]
There is an invariant that ISEQ_COVERAGE(iseq) must be Qnil if and only
if option->coverage_enabled is false. This invariant was broken by
NODE_PRELUDE which updates option->coverage_enabled but not
ISEQ_COVERAGE(iseq).
merge revision(s) 57971: [Backport #13305]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@61414 b2dd03c8-39d4-4d8f-98ff-823fe69b080e