net/http: fix panic with If-None-Match value in http.ServeContent

Fixes #39817

Change-Id: I79f2ad7c836a8a46569f603aca583fdd526d22dc
GitHub-Last-Rev: 5b88aada21
GitHub-Pull-Request: golang/go#39821
Reviewed-on: https://go-review.googlesource.com/c/go/+/239699
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/fs.go

@@ -413,6 +413,7 @@ func checkIfNoneMatch(w ResponseWriter, r *Request) condResult {
 		}
 		if buf[0] == ',' {
 			buf = buf[1:]
+			continue
 		}
 		if buf[0] == '*' {
 			return condFalse

src/net/http/fs_test.go

@@ -849,6 +849,15 @@ func TestServeContent(t *testing.T) {
 			wantStatus:      200,
 			wantContentType: "text/css; charset=utf-8",
 		},
+		"if_none_match_malformed": {
+			file:      "testdata/style.css",
+			serveETag: `"foo"`,
+			reqHeader: map[string]string{
+				"If-None-Match": `,`,
+			},
+			wantStatus:      200,
+			wantContentType: "text/css; charset=utf-8",
+		},
 		"range_good": {
 			file:      "testdata/style.css",
 			serveETag: `"A"`,