bug#32895: file progress reporter crashes on small files

[Ricardo Wurmus]

Downloading small files (such as cabal revisions) sometimes fails with a
backtrace like this:

--8<---------------cut here---------------start------------->8---
Starting download of 
/gnu/store/plr3hf9gjdcg2qhi3x4k2wjm8zajqibc-ghc-cryptohash-md5-0.11.100.1-2.cabal
>From 
>https://hackage.haskell.org/package/cryptohash-md5-0.11.100.1/revision/2.cabal...
downloading from 
https://hackage.haskell.org/package/cryptohash-md5-0.11.100.1/revision/2.cabal...
Backtrace:                                                     7.6MiB/s 00:00 | 
3KiB transferred
In ice-9/boot-9.scm:
    829:9 19 (catch _ _ #<procedure 7f8718d34458 at guix/ui.scm:615â¦> â¦)
    829:9 18 (catch _ _ #<procedure 7f8718d34470 at guix/ui.scm:733â¦> â¦)
In guix/scripts/build.scm:
   749:24 17 (_)
In ice-9/boot-9.scm:
    829:9 16 (catch _ _ #<procedure 2a53440 at ice-9/boot-9.scm:104â¦> â¦)
In guix/ui.scm:
    409:6 15 (_)
In guix/scripts/build.scm:
    675:5 14 (_)
In srfi/srfi-1.scm:
   679:15 13 (append-map _ _ . _)
   592:17 12 (map1 (#<package address@hidden gnu/packages/bioinformatâ¦>))
In guix/scripts/build.scm:
   680:31 11 (_ _)
In guix/packages.scm:
   881:14 10 (cache! #<weak-table 782/883> #<package address@hidden gâ¦> â¦)
In unknown file:
           9 (_ #<procedure thunk ()> #<procedure list _> #<undefined>)
In guix/grafts.scm:
    303:4  8 (graft-derivation #<build-daemon 256.98 2a54f50> #<derâ¦> â¦)
    181:4  7 (references-oracle #<build-daemon 256.98 2a54f50> #<derâ¦>)
   190:20  6 (_ _ _)
In guix/store.scm:
  1087:15  5 (_ #<build-daemon 256.98 2a54f50> _ _)
   661:13  4 (process-stderr _ _)
In unknown file:
           3 (display "@ download-progress /gnu/store/plr3hf9gjdcg2â¦" â¦)
In guix/status.scm:
   457:13  2 (write! _ _ 186)
    421:6  1 (_ (download-progress "/gnu/store/plr3hf9gjdcg2qhi3â¦" â¦) â¦)
In guix/progress.scm:
   214:25  0 (display-download-progress "2.cabal" #f #:start-time _ # â¦)

guix/progress.scm:214:25: In procedure display-download-progress:
In procedure /: Wrong type argument in position 1: #f
--8<---------------cut here---------------end--------------->8---

This happens because “transferred” in the “else” branch of (@ (guix
progress) display-download-progress) is #f instead of a number.  This
could be the result of running string->number on an invalid string in
“print-build-event”.


--
Ricardo