In gimp_file_proc_view_get_proc(), when there is no selected
procedure (which can happen, in particular, when searching the
list), return the "automatic" procedure and its corresponding name/
filter, if one exists, instead of bailing.

Additionally, in GimpFileDialog, use a match-all filter when
gimp_file_proc_view_get_proc() returns no filter, avoiding
CRITICALs/segfault.