runtime: defer statement not executed during panicking

[mdempsky]

This program should run successfully: https://play.golang.org/p/rms3jZGZcAQ

It does run successfully with gccgo, but it fails with cmd/compile. The problem is the deferred function containing "panic(4)" is never executed.

/cc @danscales @randall77

Found with mdempsky/deferfuzz.

[mdempsky]

Looks like a duplicate of #43921 actually. It's also fixed by CL 286712, PS 3.