proc.c: use RUBY_SAFE_LEVEL_MAX

* proc.c (rb_method_call_with_block): use RUBY_SAFE_LEVEL_MAX,
  which is declared in include/ruby/ruby.h since 2.1.  The safe
  level is now limited upto 3.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48580 b2dd03c8-39d4-4d8f-98ff-823fe69b080e