Fix a thinko in a recent commit

* src/buffer.c (Fkill_buffer): Fix last change.  (Bug#31138)
Reported by Mattias Engdegård <mattiase@acm.org>.
1 job for master in 53 minutes and 57 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #1168
test-all

00:53:57

 
Name Stage Failure
failed
test-all Test
Makefile:313: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:288: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:962: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1