merge r46831 partially.  these changes are from:
https://github.com/k-takata/Onigmo/commit/7abd7b29481f98eb92be786e3d33611fc7d000a0
[Bug #8716]

* include/ruby/oniguruma.h (ONIG_MAX_CAPTURE_GROUP_NUM,
  ONIGERR_TOO_MANY_CAPTURE_GROUPS): add cheking the number of capture
  groups.

* regerror.c (onig_error_code_to_format): ditto.

* regparse.c (scan_env_add_mem_entry): ditto.

* regexec.c (onig_region_copy, match_at): fix: segmation fault occurs
  when many groups are used.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@47400 b2dd03c8-39d4-4d8f-98ff-823fe69b080e