FpDbg: faster line  info access

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@42852 4005530d-fff6-0310-9dd1-cebe43e6787f
FpGDBMIDbg: Check nil

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@42853 4005530d-fff6-0310-9dd1-cebe43e6787f