attempt to fix rb_alloc_tmp_buffer2 for ALLOCV_N
This is a confusing function to my arithmetic-challenged mind,
but nobu seems alright with this. Anyways this lets me use
large values of elsize without segfaulting, and "make exam"
passes.
* include/ruby/ruby.h (rb_alloc_tmp_buffer2): attempt to fix
[ruby-core:81388][ruby-core:81391] [Bug #13595]
merge revision(s) 58902: [Backport #13595]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@59408 b2dd03c8-39d4-4d8f-98ff-823fe69b080e