db/version_set.cc: remove unnecessary checks

Fix for:

[db/version_set.cc:1219]: (style) Unsigned variable 'last_file'
 can't be negative so it is unnecessary to test it.
[db/version_set.cc:1234]: (style) Unsigned variable 'first_file'
 can't be negative so it is unnecessary to test it.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
ttl/ttl_test.cc: pass const string param by reference

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