bug#29274: guix pull throws exception: wrong type argument in struct_vtable

[Marco van Hulten <marco@hulten.org>]

I am trying to upgrade GuixSD, originally installed from the 0.13.0
amd64 USB image.  However, the `guix pull' command exits badly.

```
root@watson ~# guix package -u
substitute: updating list of substitutes from 'https://mirror.hydra.gnu.org'... 100.0%
substitute: updating list of substitutes from 'https://mirror.hydra.gnu.org'... 100.0%
The following package will be upgraded:
   guile-git    0.0-4.951a32c → 0.0-4.951a32c   /gnu/store/cjg6sngjq540k38s5l3nfxpydybhpas0-guile-git-0.0-4.951a32c

nothing to be done
root@watson ~# guix pull
Updating from Git repository at 'https://git.savannah.gnu.org/git/guix.git'...
Backtrace:
          10 (primitive-load "/gnu/store/qzw8q4mbk5dymmkffi6k13jphzm…")
In guix/ui.scm:
  1378:12  9 (run-guix-command _ . _)
In ice-9/boot-9.scm:
    837:9  8 (catch _ _ #<procedure 7f1fa9b05b60 at guix/ui.scm:449…> …)
    837:9  7 (catch _ _ #<procedure 7f1fa9b05b78 at guix/ui.scm:542…> …)
    837:9  6 (catch _ _ #<procedure 7f1fa6819bc8 at guix/scripts/pu…> …)
In guix/scripts/pull.scm:
   272:27  5 (_)
In guix/git.scm:
   115:29  4 (latest-repository-commit #<build-daemon 256.97 33dcf50> …)
In ice-9/boot-9.scm:
    849:4  3 (with-throw-handler _ _ _)
In git/clone.scm:
    44:14  2 (_ _ _ _)
In git/structs.scm:
   398:26  1 (clone-options->pointer #<pointer 0x0>)
In ice-9/boot-9.scm:
   760:25  0 (dispatch-exception _ _ _)

ice-9/boot-9.scm:760:25: In procedure dispatch-exception:
ice-9/boot-9.scm:760:25: In procedure struct_vtable: Wrong type argument in position 1 (expecting struct): #<pointer 0x0>
```

-Marco