[OTFC] fix TestOTFCEmptyDelete testcase

Summary: commit 08df6cb16567b0f6d8b5ffd4f44b6dc9b97ce895 broke TestOTFCEmptyDelete test case. This diff fixes it.

Test Plan:
  $ go test -v github.com/gow/otfc/
  === RUN TestOTFCStructSize
  --- PASS: TestOTFCStructSize (0.00 seconds)
  === RUN TestOTFCSingleSetAndGet
  --- PASS: TestOTFCSingleSetAndGet (0.00 seconds)
  === RUN TestOTFCSequentialMultipleSets
  2013/07/02 02:04:00 Testing multiple sets with 887 samples (seed: 1372755840)
  --- PASS: TestOTFCSequentialMultipleSets (0.08 seconds)
  === RUN TestOTFCSequentialMultipleSetsGetsDeletes
  2013/07/02 02:04:00 Testing multiple sets & gets with 518 samples (seed: 32)
  --- PASS: TestOTFCSequentialMultipleSetsGetsDeletes (0.02 seconds)
  === RUN TestOTFCEmptyGet
  --- PASS: TestOTFCEmptyGet (0.00 seconds)
  === RUN TestOTFCDoubleSets
  --- PASS: TestOTFCDoubleSets (0.00 seconds)
  === RUN TestOTFCMaxIndexCapacity
  --- PASS: TestOTFCMaxIndexCapacity (0.02 seconds)
  === RUN TestOTFCEmptyDelete
  2013/07/02 02:04:00 Testing Empty delete
  2013/07/02 02:04:00 Random seed value: [1372755840]
  --- PASS: TestOTFCEmptyDelete (0.00 seconds)
  === RUN TestOTFCDeleteNonExistingKey
  2013/07/02 02:04:00 Testing deletion of non existing keys
  2013/07/02 02:04:00 Random seed value: [1372755840]
  --- PASS: TestOTFCDeleteNonExistingKey (0.03 seconds)
  PASS
  ok    github.com/gow/otfc 0.164s

Reviewers: tommy.testy

Reviewed By: tommy.testy

Differential Revision: http://dev.chetan.io/D41