libgimp: add precondition checks for gimp_is_canonical_identifier()

on all procedure name parameters.
3 jobs for master in 20 minutes and 17 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Babl
passed #396023
babl-git-meson

00:02:08

 
  Gegl
passed #396024
gegl-git-meson

00:05:41

 
  Gimp
passed #396025
build-all-git

00:12:28