cmd/vet: detect defer resp.Body.Close() before error check

cmd/vet: detect defer resp.Body.Close() before error check

This check detects the code

	resp, err := http.Get("http://foo.com")
	defer resp.Body.Close()
	if err != nil {
		...
	}

For every call to a function on the net/http package or any method
on http.Client that returns (*http.Response, error), it checks
whether the next line is a defer statement that calls on the response.

Fixes #17780.

Change-Id: I9d70edcbfa2bad205bf7f45281597d074c795977
Reviewed-on: https://go-review.googlesource.com/32911
Reviewed-by: Rob Pike <r@golang.org>