Ruby

Fix Issues reported by PVS-Studio static analyzer

• vm.c (vm_set_main_stack): remove unnecessary check. toplevel
  binding must be initialized. [Bug #12611] (N1)
• win32/win32.c (w32_symlink): fix return type. [Bug #12611] (N3)
• string.c (rb_str_split_m): simplify the condition.
  Bug #12611

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55729 b2dd03c8-39d4-4d8f-98ff-823fe69b080e