bignum.c: fix rb_cstr_parse_inum endp

	* bignum.c (rb_cstr_parse_inum): stores the address of the first
	  invalid character when str is too big or contains an underscore.
	bignum.c: unnecessary check

	* bignum.c (rb_cstr_parse_inum): remove unnecessary check.
	  successive sign is rejected by conv_digit.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@57953 b2dd03c8-39d4-4d8f-98ff-823fe69b080e