gc.c: check numeric string

* gc.c (get_envparam_int, get_envparam_double): check invalid string
  as numeric.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44865 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
Merge branch 'trunk' of https://github.com/ruby/ruby into trunk