Flymake: protect against problematic invalid diagnostics

Flymake: protect against problematic invalid diagnostics

If a backend reports a diagnostic which is out of bounds, it still
lives in flymake--state and will still be cleaned up in the next run
of flymake--publish-diagnostics.  But if flymake--highlight-line
doesn't give it an overlay (which it didn't until now), things will
break afterwards.

See bug#66759 and https://github.com/joaotavora/eglot/discussions/1311

* lisp/progmodes/flymake.el (flymake--highlight-line): Set
flymake--diag-overlay earlier.
(Version): Bump to 1.3.7.