Fix use-after-free deleting an in-use printer.

[andreer]

create_local_bg_thread() holds a reference to the printer via printer->use while it generates the IPP Everywhere PPD. cupsdDeleteTemporaryPrinters() respected that reference, but cupsdDeletePrinter() did not, so an explicit CUPS-Delete-Printer (as cups-browsed issues when it replaces a discovered queue) freed the printer mid-thread and corrupted the heap ("corrupted double-linked list").

Defer deletion while printer->use > 0 by flagging the printer under printer->lock; cupsdDeleteTemporaryPrinters() then reaps it once the thread releases its reference. Extends the use-count protection from #1655 to the explicit deletion path.

[andreer]

CI may need this first #1723

[michaelrsweet]

I think we can simply this to just decrement use to 0 but not immediately delete it. Will think some, and we should bring this fix to 2.4.x as well.

[michaelrsweet]

OK, my fixes:

[master 4bfa89a] Fix potential scheduler printer use-after-free bug (Issue #1722)
[master c1ce727] Minor logic fix for (Issue #1722)

[2.4.x eb06fe2] Fix potential scheduler printer use-after-free bug (Issue #1722)
[2.4.x 2074ceb] Minor logic fix for (Issue #1722)

Basically, I initialize "use" to 1 in cupsdAddPrinter, have cupsdDeletePrinter decrement "use" and only delete when it gets to 0, and update the cupsdDeleteTemporaryPrinters to actually delete any printer whose use count is 0 or a temporary printer whose use count is 1 and it needs to go.