Checking for methodset existance

[marco6]

This PR adds a check for the existence of the methodset in the interpreter implementation of typeasserts. This is necessary since this field is optional.

This should be the last bit that fixes #3788 since pull #3986 only covers XXX.(any) cases and still crashes if the interface has methods. As such, it should also fix #4079.

As a last note, I also tried a different approach here where I forced the methodset to be mandatory as I felt it was more robust, but the simplicity of these 4 lines seems to be unmatched.

[dgryski]

/cc @aykevl

[TheSignPainter98]

Any progress on this? I'm also having this issue