Fix lisp indent infloop on unfinished strings (Bug#37045)

* lisp/emacs-lisp/lisp-mode.el (lisp-indent-calc-next): Stop trying to
skip over strings if we've hit the end of buffer.
* test/lisp/emacs-lisp/lisp-mode-tests.el
(lisp-indent-unfinished-string): New test.
1 job for emacs-26 in 25 minutes and 32 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #3016
test

00:25:32

 
Name Stage Failure
failed
test Test
make[2]: *** [check-doit] Error 1
Makefile:285: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:255: recipe for target 'check' failed
make[1]: *** [check] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:951: recipe for target 'check' failed
make: *** [check] Error 2
ERROR: Job failed: exit code 1