Don't assert for unexpected FX slot IDs

Although they should never happen at this point and would indicate a serious
problem, there's no need to kill the process instead of returning a dummy value
(and the assert wouldn't check in release builds anyway).

Don't assert for unexpected FX slot IDs

Although they should never happen at this point and would indicate a serious
problem, there's no need to kill the process instead of returning a dummy value
(and the assert wouldn't check in release builds anyway).

[EAX] Fix error handling (#953)

- Set error on EAX call failure if context is available.
- Reset error on successful retreiving the last error code.

Reference: EAX 4.0 - Programmer's Guide (pg.37)