Don't clobber match data in timer

[darkfeline]

Fixes #1805

[CLAassistant]

All committers have signed the CLA.

[cpitclaudel]

Thanks, excellent catch. Maybe we should actually put this in flycheck-buffer-automatically, to catch similar errors that may pop up from elsewhere?

[darkfeline]

I'm not sure. The Emacs manual states that all functions are allowed to change match data, so there's nothing specifically wrong with flycheck-buffer-automatically overwriting match data. It's only timer functions that need to be careful. (I don't know why Emacs doesn't automatically save match data for timers like it does for process filters.)

[cpitclaudel]

It's only timer functions

And change hooks, right? (we already have a save-match-data for that case). The fact that we had a save-match-data in one of the callers but missed it in the other (idle-trigger) suggests to me that we should just harden flycheck-buffer-automatically once and for all

[darkfeline]

Okay, sure. I'm keeping the save-match-data in the timer and change hooks though, since I'm paranoid something may get added there in the future that changes match data again.

[cpitclaudel]

OK, works for me :)