Ruby

• iseq.c, vm_eval.c: set th->base_block properly.
  th->base_block is information for (a) parsing, (b) compiling
  and (c) setting up the frame to execute the program passed by
  `eval' method. For example, (1) parser need to know up-level
  variables to detect it is variable or method without paren.
  Befor (a), (b) and (c), VM set th->base_block by passed bindng
  (or previous frame information). After execute (a), (b) and (c),
  VM should clear th->base_block. However, if (a), (b) or (c)
  raises an exception, then th->base_block is not cleared.
  Problem is that the uncleared value th->balo_block is used for
  irrelevant iseq compilation. It causes SEGV or critical error.
  I tried to solve this problem: to clear them before exception,
  but finally I found out that it is difficult to do it (Ruby
  program can be run in many places).
  Because of this background, I set th->base_block before
  compiling iseq and restore it after compiling.
  Basically, th->base_block is dirty hack (similar to global
  variable) and this patch is also dirty.
• bootstraptest/test_eval.rb: add a test for above.
• internal.h: remove unused decl.
• iseq.c (rb_iseq_compile_with_option): add base_block parameter.
  set th->base_block before compation and restore it after
  compilation.
• ruby.c (require_libraries): pass 0 as base_block instead of
  setting th->base_block
• tool/compile_prelude.rb (prelude_eval): apply above changes.
• vm.c, vm_eval.c: ditto.
• vm_core.h: add comments.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36179 b2dd03c8-39d4-4d8f-98ff-823fe69b080e