db/db_impl.cc: fix object handling, remove double lines

Fix for:

[db/db_impl.cc:4039]: (error) Instance of 'StopWatch' object is
 destroyed immediately.
[db/db_impl.cc:4042]: (error) Instance of 'StopWatch' object is
 destroyed immediately.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
util/signal_test.cc: suppress intentional null pointer deref

Add comment to enabele cppcheck suppression of intentional null
pointer deref via --inline-suppr option.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
util/ldb_cmd.cc: prefer prefix ++operator for non-primitive types

Prefer prefix ++operator for non-primitive types like iterators for
performance reasons. Prefix ++/-- operators avoid creating a temporary
copy.

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