flag: recover panic when calling String on zero value in PrintDefaults
When printing the usage message, recover panics when calling String
methods on reflect-constructed flag.Value zero values. Collect the panic
messages and include them at the end of the PrintDefaults output so that
the programmer knows to fix the panic.
Fixes #28667
Change-Id: Ic4378a5813a2e26f063d5580d678add65ece8f97
Reviewed-on: https://go-review.googlesource.com/c/go/+/396574
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Trust: Andrew Gerrand <adg@golang.org>