GlobalFunctions: Fix 'Undefined index' notice in wfBacktrace

Notice: Undefined index: line in
/var/www/wiki/includes/GlobalFunctions.php on line 1840

Caused by fc2e6df64ffabe9236b8b5eee43d3d89e235670a, previous versions
used isset here.

Change-Id: Ie9434f09e0905346ace19909fb316d47c75e0440
Merge "GlobalFunctions: Fix 'Undefined index' notice in wfBacktrace"