bug#37507: 26.3; Non-number value for `fill-column' causes wrong-type-arg error for `C-h f'

[Drew Adams]

`fill-column' is supposed to have an integer value > 0.

But if it has a non-numeric value then other functions can barf, unless
they watch out for that.

E.g.

emacs -Q

(setq fill-column nil)

C-h f forward-char ; raises error

That happens because `describe-function-1` calls
`fill-region-as-paragraph`, which ultimately calls
`fill-match-adaptive-prefix`, which barfs when it tries to add the
result of `current-fill-column` (nil) to integers.

This is a regression from older releases.


In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.17763
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''