fix(connection): panic in startWatcher

[lyyourc]

Description

I found a panic in my apps:

panic: sync/atomic: store of nil value into Value
	/usr/local/go/src/sync/atomic/value.go:47 +0x12c
	/go/pkg/mod/github.com/go-sql-driver/mysql@v1.6.0/utils.go:831
	/go/pkg/mod/github.com/go-sql-driver/mysql@v1.6.0/connection.go:435 +0x51
	/go/pkg/mod/github.com/go-sql-driver/mysql@v1.6.0/connection.go:622 +0x19a
        /go/pkg/mod/github.com/go-sql-driver/mysql@v1.6.0/connection.go:611 +0xbe

But according the document, ctx.Err() should not be nil, after ctx.Done() is emitted:

// If Done is not yet closed, Err returns nil.
// If Done is closed, Err returns a non-nil error explaining why:
// Canceled if the context was canceled
// or DeadlineExceeded if the context's deadline passed.
// After Err returns a non-nil error, successive calls to Err return the same error.
Err() [error](https://pkg.go.dev/builtin#error)

I CAN NOT UNDERSTAND WHY.

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[methane]

I don't want to merge this unless we understand what is happend, or it is reproducible at least.