[LOG] avoid unnecessary object creation if log level is not enabled

Just drawing two lines with the pencil tool resulted in about 1 MB log
file so there is much of data that only be created if the log level is
enabled.

Change-Id: I46930f2fc6d3b78b9680e82e8a81c5b9719fa05c
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1348
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>