db/version_set.cc: use !empty() instead of 'size() > 0'

Use empty() since it should be prefered as it has, following
the standard, a constant time complexity regardless of the
containter type. The same is not guaranteed for size().

Fix for:
[db/version_set.cc:2250]: (performance) Possible inefficient
 checking for 'column_families_not_found' emptiness.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
table/cuckoo_table_reader.cc: pass func parameter by ref

Fix for:

[table/cuckoo_table_reader.cc:198]: (performance) Function
 parameter 'file_data' should be passed by reference.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
table/table_test.cc: pass func parameter by reference

Fix for:

[table/table_test.cc:1218]: (performance) Function parameter
 'prefix' should be passed by reference.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
db/db_test.cc: fix make check

Fix compiler warning:

In file included from db/db_test.cc:44:0:
./util/testharness.h: In instantiation of ‘rocksdb::test::Tester&
 rocksdb::test::Tester::IsEq(const X&, const Y&) [with X = long
 unsigned int; Y = int; rocksdb::test::Tester =
 rocksdb::test::Tester]’:
db/db_test.cc:1352:3:   required from here
./util/testharness.h:93:3: error: comparison between signed and
 unsigned integer expressions [-Werror=sign-compare]

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
ldb_cmd_execute_result.h: perform init in initialization list

Fix for:

[util/ldb_cmd_execute_result.h:18]: (performance) Variable 'message_'
 is assigned in constructor body. Consider performing initialization
 in initialization list.
[util/ldb_cmd_execute_result.h:23]: (performance) Variable 'message_'
 is assigned in constructor body. Consider performing initialization
 in initialization list.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>