src/kv/MemDB.cc: the type of the parameter of push_back() does not match the ops...

src/kv/MemDB.cc: the type of the parameter of push_back() does not match the ops's value_type

 - NULL is a C-type usage, which is allow by GCC.
   But Clang is more strict in checking.

Error looks like:
src/kv/MemDB.cc:228: error: no matching member function
for call to 'push_back'
  ops.push_back(make_pair(DELETE,
  ~~~~^~~~~~~~~
/usr/include/c++/v1/vector:685:36: note: candidate function not viable:
no known conversion from 'pair<[...], pair<[...], nullptr_t>>' to
      'const pair<[...], pair<[...], ceph::buffer::list>>' for 1st argument
    _LIBCPP_INLINE_VISIBILITY void push_back(const_reference __x);
                                   ^

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>