fmt: clear width and precision when recovering formatting object from the pool

Probably a day 1 oversight, and almost always inconsequential, but
there is evidence of occasional trouble. There is no reason not to
clear them.

I tried and failed to write a test to catch this, but the change should
be harmless and is all but certain to fix the problem.

Fixes #61913

Change-Id: I0f7bbb4ab2780d8999d3ff7a35255dc07fb5c7e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/556215
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/fmt/format.go

@@ -53,6 +53,8 @@ type fmt struct {
 
 func (f *fmt) clearflags() {
 	f.fmtFlags = fmtFlags{}
+	f.wid = 0
+	f.prec = 0
 }
 
 func (f *fmt) init(buf *buffer) {