M-x elp-instrument-package RET RET makes emacs unusable

[Simon Josefsson]

I wanted to instrument a package for profiling, and typed M-x
elp-instrument-packe RET and accidently hit one RET extra, thus
presumably trying to instrument all elisp functions.  After this emacs
is completely unusable, I can't even kill the window:

Lisp nesting exceeds max-lisp-eval-depth

I'm not sure if it's possible to prevent this properly, but it's quite
annoying if you had lots of work in progress.

(Happens in both 20.7 and 21.0 pretest)