bug#22914: "VM stack overflow" with graft dependencies from substitutes

[Eric Bavier <ericbavier@openmailbox.org>]

Starting with commit c90cb5c9d84ded26ef44d1e6593508d5b9e4655e (via 'git
bisect') I get the following error messages after `make clean && make
&& ./pre-inst-env guix environment texlive`:

--- BEGIN SNIP ---
Backtrace:
In srfi/srfi-1.scm:
 646: 19 [append-map #<procedure dependency-grafts (item)> #]
 578: 18 [map #<procedure dependency-grafts (item)> #]
In guix/grafts.scm:
 235: 17 [cumulative-grafts # # # ...]
In srfi/srfi-1.scm:
 646: 16 [append-map #<procedure dependency-grafts (item)> #]
 578: 15 [map #<procedure dependency-grafts (item)> #]
In guix/grafts.scm:
 235: 14 [cumulative-grafts # # # ...]
In srfi/srfi-1.scm:
 646: 13 [append-map #<procedure dependency-grafts (item)> #]
 578: 12 [map #<procedure dependency-grafts (item)> #]
In guix/grafts.scm:
 235: 11 [cumulative-grafts # # # ...]
In srfi/srfi-1.scm:
 646: 10 [append-map #<procedure dependency-grafts (item)> #]
 578: 9 [map #<procedure dependency-grafts (item)> #]
In guix/grafts.scm:
 222: 8 [dependency-grafts "/gnu/store/09gwygs9vdja1bkpyfqsb51rjzs7d5zs-libunistring-0.9.6"]
 156: 7 [item->deriver # ...]
In guix/store.scm:
 832: 6 [valid-derivers # ...]
In srfi/srfi-1.scm:
 534: 5 [unfold #<procedure 6ecb980 at guix/serialization.scm:162:12 (t-722807)> ...]
In guix/serialization.scm:
 122: 4 [read-string #<input-output: socket 10>]
 114: 3 [read-byte-string #<input-output: socket 10>]
  85: 2 [read-int #<input-output: socket 10>]
  71: 1 [get-bytevector-n* #<input-output: socket 10> 8]
In unknown file:
   ?: 0 [get-bytevector-n #<input-output: socket 10> 8]

ERROR: In procedure get-bytevector-n:
ERROR: Throw to key `vm-error' with args `(vm-run "VM: Stack overflow" ())'.
--- END SNIP ---

This backtrace is printed 3 times.

If it means anything, it's been at least a week since I've fetched any
substitutes from hydra on this machine.

`~Eric