vm_core.h: suppress warnings

* vm_core.h (UNINITIALIZED_VAR): suppress warnings by clang 4.2.
  [ruby-core:51742] [Bug #7756]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39855 b2dd03c8-39d4-4d8f-98ff-823fe69b080e