re.c: check if onig_region_copy failed

* re.c (CHECK_REGION_COPIED): onig_region_copy() can fail when
  memory exhausted but returns nothing, so check by if allocated.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48669 b2dd03c8-39d4-4d8f-98ff-823fe69b080e