add assertions for division NaN

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47023 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
numeric.c: 0 % Float::NAN returns Float::NAN

* numeric.c (flodivmod): all results are NaN if divisor is NaN.
  [fix GH-692]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47024 b2dd03c8-39d4-4d8f-98ff-823fe69b080e