[llvm] [Offload][OpenMP] Fix RPC callback registration when RPC server is not initialized (PR #210215)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> So I think, the asserting `getRPCServer()` is the wrong tool here; a null‑safe accessor + guard is the minimal correct fix.

The plugin reporting that it is initialized and then having things that are not initialized is violaation, that's why it's an assert and the pointer is not exposed by the API directly. My concern is that if we just 'made this work' in this case then you'd just end up with cases where the interface was *not* properly initialized. The fortran runtime is trying to initialize a callback, if we just drop that silently, then those will be unhandled and the interface will error if they actually get used.

https://github.com/llvm/llvm-project/pull/210215