Remove a check for merge operator in builder.cc

Summary:
Previously, builder.cc has a check for merge operator which prevents
RocksDB from crash when reopening a DB w/o properly specifying the merge
operator.  However, currently we observed a memory leak on failing in
RocksDB recovery.  This diff removes such check and let it crash instead of
causing memory leak for now before we have identified the real cause of
the memory leak.

Test Plan: make all check

Reviewers: sdong

Subscribers: ljin, igor

Differential Revision: https://reviews.facebook.net/D20913
Fixed a warning / error in signed and unsigned comparison

Summary:
Fixed the following compilation error detected in mac:
db/db_test.cc:2524:3: note: in instantiation of function template
  specialization 'rocksdb::test::Tester::IsEq<unsigned long long, int>' requested here
    ASSERT_EQ(int_num, 0);
      ^

Test Plan:
make
Fixing broken Mac build

Summary: Made some small changes to fix the broken mac build

Test Plan: make check all in both linux and mac. All tests pass.

Reviewers: sdong, igor, ljin, yhchiang

Reviewed By: ljin, yhchiang

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20895