net/mail: avoid panic in (*Address).String for malformed addresses.

Fixes #12098.

Change-Id: I190586484cd34856dccfafaba60eff0197c7dc20
Reviewed-on: https://go-review.googlesource.com/13500Reviewed-by: Rob Pike <r@golang.org>

src/net/mail/message.go

@@ -171,7 +171,14 @@ func (a *Address) String() string {
 
 	// Format address local@domain
 	at := strings.LastIndex(a.Address, "@")
+	var local, domain string
+	if at < 0 {
+		// This is a malformed address ("@" is required in addr-spec);
+		// treat the whole address as local-part.
+		local = a.Address
+	} else {
 		local, domain := a.Address[:at], a.Address[at+1:]
+	}
 
 	// Add quotes if needed
 	// TODO: rendering quoted local part and rendering printable name

src/net/mail/message_test.go

@@ -483,6 +483,14 @@ func TestAddressFormatting(t *testing.T) {
 			&Address{Name: "Böb Jacöb", Address: "bob@example.com"},
 			`=?utf-8?q?B=C3=B6b_Jac=C3=B6b?= <bob@example.com>`,
 		},
+		{ // https://golang.org/issue/12098
+			&Address{Name: "Rob", Address: ""},
+			`"Rob" <>`,
+		},
+		{ // https://golang.org/issue/12098
+			&Address{Name: "Rob", Address: "@"},
+			`"Rob" <@>`,
+		},
 	}
 	for _, test := range tests {
 		s := test.addr.String()