merge r46831 partially. extracted commits are as follows.
https://github.com/k-takata/Onigmo/commit/b9fba1dc63ccb42a86e934011b468e6022fabb74
https://github.com/k-takata/Onigmo/commit/c1fc76b9bd463948ffc5058bc352bf93732f0314
https://github.com/k-takata/Onigmo/commit/a0efc0a200f7108ca3d5ac3039c8f952e0051619
https://github.com/k-takata/Onigmo/commit/c7cda4ed5676167b0d01bb5555724f6164fbdb13
[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_1@47223 b2dd03c8-39d4-4d8f-98ff-823fe69b080e