shm: remove wl_shm_buffer.pool NULL checks

wl_shm_buffer.pool is never set to NULL. The only time it's set is
in shm_pool_create_buffer, and the pool is guaranteed to be non-NULL
there.

Signed-off-by: Simon Ser <contact@emersion.fr>
3 jobs for shm-buffer-pool-null in 1 minute and 11 seconds (queued for 4 minutes and 19 seconds)
Status Job ID Name Coverage
  Review
passed #10714471
check-commit

00:00:13

 
  Prep
passed #10714472
debian:buster@container-prep

00:00:29

 
  Build
passed #10714473
build-native

00:00:28