bug#38493: Crash in procedure display-download-progress

[Leo Famulari]

I just had `guix package` crash while downloading substitutes from an
nginx mirror. The command succeeded when I ran it again.

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$ guix describe
Generation 106  Dec 04 2019 10:41:03    (current)
  guix 8548b3d
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: 8548b3dd87a795f850a97add59fd87ec5f49f092

GUIX_PACKAGE_PATH="/home/leo/pkgs"
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$ guix package -u . --do-not-upgrade=blender --do-not-upgrade=audacity
substitution of /gnu/store/gfpiq0nnv7wrbq5mdydyj5lkwa4hbbxk-ghc-8.6.5 complete
substitution of /gnu/store/5lxzblrjzaw5rmjcb263zszvzx46bdw5-mono-4.4.1.0 
complete
 openjdk-12.33-jdk  157.2MiB                         784KiB/s 01:36 [########   
       ]  46.9%Backtrace:
In ice-9/boot-9.scm:
    829:9 19 (catch _ _ #<procedure 7f903cb97380 at guix/ui.scm:650…> …)
    829:9 18 (catch _ _ #<procedure 7f903cb97398 at guix/ui.scm:773…> …)
In guix/scripts/package.scm:
   943:10 17 (_)
In guix/status.scm:
    768:4 16 (call-with-status-report _ _)
In guix/scripts/package.scm:
   951:14 15 (_)
In guix/build/syscalls.scm:
   1142:4 14 (call-with-file-lock/no-wait _ _ _)
In guix/scripts/package.scm:
   889:21 13 (_)
    601:4 12 (options->installable ((do-not-upgrade . "audacity") …) …)
In srfi/srfi-1.scm:
   466:18 11 (fold #<procedure 7f903ac7ff60 at guix/scripts/package…> …)
In guix/scripts/package.scm:
   245:39 10 (transaction-upgrade-entry _ #<<manifest-transaction> i…>)
In guix/packages.scm:
   948:16  9 (cache! #<weak-table 1709/3517> #<package diffoscope@1…> …)
In guix/grafts.scm:
    323:4  8 (graft-derivation #<store-connection 256.99 7f903e80fa…> …)
    172:4  7 (references-oracle #<store-connection 256.99 7f903e80f…> …)
   182:20  6 (_ _ _)
In guix/store.scm:
  1222:15  5 (_ #<store-connection 256.99 7f903e80fae0> _ _)
   718:13  4 (process-stderr _ _)
In unknown file:
           3 (display "@ substituter-succeeded /gnu/store/azd3rg350…" …)
In guix/status.scm:
   691:16  2 (write! _ _ _)
    605:6  1 (_ (download-progress "/gnu/store/7i0vxbrjcrib4aw20…" …) …)
In guix/progress.scm:
   219:14  0 (display-download-progress "7i0vxbrjcrib4aw20bdawz4q96…" …)

guix/progress.scm:219:14: In procedure display-download-progress:
In procedure =: Wrong type: #f
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