bigdecimal.c: check underflow
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36854 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
bigdecimal.c: check underflow
strtod() sets errno to ERANGE at underflow too. [ruby-core:47342]
[Bug #6944]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36854 b2dd03c8-39d4-4d8f-98ff-823fe69b080e