Crash when leaving session with invalid bookmarks or breakpoints

To reproduce:

• Open session A containing project P
• Add a bookmark or a breakpoint near the end of a source file, e. g. in line 50.
• Switch to session B and open project P there
• Remove lines from the source file until it is shorter than the bookmark's position, e. g. reduce it to 45 lines. Save.
• Switch back to session A.
• Try to exit Creator.