fix RSTRUCT_LEN macro in public C API
rb_struct_size returns an Integer VALUE, so it must be converted
to a `long` for compatibility with previous Ruby C API versions.
* ext/-test-/struct/len.c: new
* test/-ext-/struct/test_len.rb: new
* include/ruby/ruby.h (RSTRUCT_LEN): use NUM2LONG
[ruby-core:80692] [Bug #13439]
merge revision(s) 58359: [Backport #13439]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@58636 b2dd03c8-39d4-4d8f-98ff-823fe69b080e