Fix memory leak when parsing invalid pattern matching

If the pattern matching is invalid, then the pvtbl would get leaked. For
example:

    10.times do
      100_000.times do
        eval(<<~RUBY)
          case {a: 1}
          in {"a" => 1}
          end
        RUBY
      rescue SyntaxError
      end

      puts `ps -o rss= -p #{$$}`
    end

Before:

    28096
    44768
    61472
    78512
    94992
    111504
    128096
    144528
    161008
    177472

After:

    14096
    14112
    14112
    14176
    14208
    14240
    14240
    14240
    14240
    14240

0 files changed, 0 insertions, 0 deletions