Fixes crash when debugger session is disposed

Fixes VSTS #801713

It was noticed while looking at this stack trace that it was possible for
protocolClient to be null when the session was disposed.

```
[ERROR] FATAL UNHANDLED EXCEPTION: System.NullReferenceException: Object reference not set to an instance of an object
  at MonoDevelop.Debugger.VsCodeDebugProtocol.VSCodeDebuggerSession.UpdateBreakpoints () [0x00191] in /Users/vsts/agent/2.147.1/work/1/s/monodevelop/main/src/addins/MonoDevelop.Debugger.VSCodeDebugProtocol/MonoDevelop.Debugger.VsCodeDebugProtocol/VSCodeDebuggerSession.cs:388
  at MonoDevelop.Debugger.VsCodeDebugProtocol.VSCodeDebuggerSession.OnUpdateBreakEvent (Mono.Debugging.Client.BreakEventInfo eventInfo) [0x0003d] in /Users/vsts/agent/2.147.1/work/1/s/monodevelop/main/src/addins/MonoDevelop.Debugger.VSCodeDebugProtocol/MonoDevelop.Debugger.VsCodeDebugProtocol/VSCodeDebuggerSession.cs:442
  at Mono.Debugging.Client.DebuggerSession.UpdateBreakEvent (Mono.Debugging.Client.BreakEvent be) [0x00021] in /Users/vsts/agent/2.147.1/work/1/s/monodevelop/main/external/debugger-libs/Mono.Debugging/Mono.Debugging.Client/DebuggerSession.cs:687
  at Mono.Debugging.Client.DebuggerSession+<>c__DisplayClass116_0.<OnBreakpointModified>b__0 () [0x0000d] in /Users/vsts/agent/2.147.1/work/1/s/monodevelop/main/external/debugger-libs/Mono.Debugging/Mono.Debugging.Client/DebuggerSession.cs:722
  at Mono.Debugging.Client.DebuggerSession.<Dispatch>b__92_0 (System.Object <p0>) [0x00054] in /Users/vsts/agent/2.147.1/work/1/s/monodevelop/main/external/debugger-libs/Mono.Debugging/Mono.Debugging.Client/DebuggerSession.cs:300
```

1 files changed, 0 insertions, 0 deletions

diff --git a/main/external/debugger-libs b/main/external/debugger-libs
-Subproject b199056691fbb5e9c0a9b3ac1e020e946997f24
+Subproject 8d8e8d14f76a20c24fbe612a227344eba678ba6