When the underlying transport returns a network error, the RoundTrip
method returns (nil, error). The defer was attempting to access resp
without checking if it was nil first, causing a panic. Fix this by
checking for nil in the defer.
Also changes driveTransport.tr from *http.Transport to http.RoundTripper
and adds a test.
Fixes#17306
Signed-off-by: Andrew Dunham <andrew@tailscale.com>
Change-Id: Icf38a020b45aaa9cfbc1415d55fd8b70b978f54c
698eecda04