Monitor: Check for possibly undefined memory usage values

Signed-off-by: Ann + J.M <phpMyAdmin@ZweiSteinSoft.de>
Merge remote-tracking branch 'pma/QA_4_2'
Fix coding style

Signed-off-by: Marc Delisle <marc@infomarc.info>
Merge branch 'master' of https://github.com/phpmyadmin/phpmyadmin
rfe #1523 Better visual clue for table structure primary key column

Signed-Off-By: Ann + J.M. <phpMyAdmin@ZweiSteinSoft.de>