Reset the owner ivar in LRU#synchronize

Without this the following could happen:

1. Thread A acquires the lock and sets the ownership to A.
2. Thread A yields and returns
3. Thread B tries to acquire the lock
4. At this exact moment Thread A calls the "synchronize" method again
   and sees that the "owner" variable is still set to Thread A
5. Both thread A and B can now access the underlying data in parallel,
   possibly leading to corrupted objects

This can be demonstrated using the following script:

    require 'oga'

    lru = Oga::LRU.new(64)

    threads = 50.times.map do
      Thread.new do
        loop do
          number = rand(100)

          lru[number] = number
        end
      end
    end

    threads.each(&:join)

Run this for a while on either JRuby or Rubinius and you'll end up with
errors such as "ConcurrencyError: Detected invalid array contents due to
unsynchronized modifications with concurrent users" on JRuby or
"ArgumentError: negative array size" on Rubinius.

Resetting the owner variable ensures the above can never happen. Thanks
to @chrisseaton for bringing this up earlier today.

lib/oga/lru.rb

@@ -137,8 +137,10 @@ module Oga
       if @owner != Thread.current
         @mutex.synchronize do
           @owner = Thread.current
+          retval = yield
+          @owner = nil
 
-          yield
+          retval
         end
       else
         yield