When 'Step Over' is performed in an attached debugger, Unity crashes.

Steps to reproduce:
1) Open the attached project.
2) Open 'NewBehaviourScript.cs' in MonoDevelop or other IDE that allows debugging Unity.
3) Set a breakpoint in line 8 of 'NewBehaviourScript.cs'.
4) Attach to Unity process.
5) In the Editor, enter play mode.
6) Return to the debugger.
7) Press 'Step Over' twice.

Unity crashes in UTF16String::BorrowString(ScriptingStringPtr). This doesn't happen when stepping into/out.

Reproduced on:
5.6.0b10, 5.5.0p1