process: Talloc home_trigger dummy request

Allocate the dummy request in home_trigger with talloc, instead of
allocating it on the stack, as the rest of the code expects it to be a
valid talloc context.

This fixes a talloc_abort resulting from xlat_tokenize_request invoking
talloc_typed_strdup with the dummy request as the talloc context.

NOTE: Besides this one, master seems to have two similar cases in
src/modules/proto_bfd/bfd.c, found with "grep -r --include '*.[hc]'
'^\s*REQUEST\s\+[a-zA-Z]' *".