internal/jsonrpc2: fix invalid invocations of Fatalf in goroutines
internal/jsonrpc2: fix invalid invocations of Fatalf in goroutines

Found by running the go vet pass 'testinggoroutine'.

Change-Id: I38f17877e2a97ffb823bb97850d21107743271d7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/217179
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools