bug#70101: Buiding in high debug level crashed

bug#70101: Buiding in high debug level crashed

[Jim Dupont]

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guix build --debug=999 -v 99  -f cpuinfo.scm > error.log 2>&1
fails
http://sprunge.us/16uhk6

```In guix/store.scm:
   1403:5 10 (map/accumulate-builds #<sto\re-connection 256.99
ffffac918d70> # _ …)
  1419:15  9 (_ #<store-connection 256.99\
 ffffac918d70> _ _)                          739:14  8 (process-stderr _ _)
In unknown file:                                     7 (display "@
build-log 158510\ 45\n[ 11%] Creating directories for…" …) In
guix/status.scm:                          779:20  6 (write! _ _ _)
     In srfi/srfi-1.scm:
   460:18  5 (fold #<procedure ffffb7170e\00 at guix/status.scm:668:2
(event s…> …)
In guix/status.scm:                          669:15  4 (_ (build-log 158510
"[ 11%] Creating directories for 'googlet…") #)
   180:23  3 (update-build #<<build-statu\s> building: () downloading: ()
buil…> …) In ice-9/boot-9.scm:                        1685:16  2
(raise-exception _ #:continuable? _)
1685:16  1 (raise-exception _ #:continuable? _)
  1685:16  0 (raise-exception _ #:continu\able? _)

ice-9/boot-9.scm:1685:16: In procedure raise-exception:
In procedure struct-vtable: Wrong type argument in position 1 (expecting
struct): #f
```

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