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]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58359 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
