Crash in HandleSymbolCodeQuery if module is unloaded

[jkriegshauser]

We have a heavily modularized application. When running with privileges to get sys-tracing, the query backlog can grow quite high.

The shutdown order looks something like:

1. Unload most modules.
2. Unload our profiling-tracy module (blocks until tracy::ShutdownProfiler() returns)
3. Unload the rest of our modules
4. Exit application

Unfortunately, if a SymbolCodeQuery query is processed for a module that unloaded, we usually end up in the CrashHandler and the application is hung at this point.