fmt: avoid badverb formatting for %q when used with integers

Instead of returning a bad verb error format for runes above
utf8.Maxrune return a quoted utf8.RuneError rune (\ufffd).
This makes the behaviour consistent with the "c" verb and
aligns behaviour to not return bad verb error format when
a verb is applied to the correct argument type.

Fixes #14569

Change-Id: I679485f6bb90ebe408423ab68af16cce38816cd0
Reviewed-on: https://go-review.googlesource.com/c/go/+/248759


Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>