proto: fix govet failures in Go 1.10 (#505)

In Go 1.10, go test automatically runs go vet as well.  This
uncovered a vet failure in TestUnmarshalRepeatingNonRepeatedExtension
where pb.ComplexExtension is passed as a fmt.Stringer to t.Errorf.
The string method exists only on the pointer-receiver type,
*pb.ComplexExtension.

proto/extensions_test.go

@@ -623,7 +623,7 @@ func TestUnmarshalRepeatingNonRepeatedExtension(t *testing.T) {
 			t.Fatalf("[%s] Invalid extension", test.name)
 		}
 		if !proto.Equal(ext, &want) {
-			t.Errorf("[%s] Wrong value for ComplexExtension: got: %s want: %s\n", test.name, ext, want)
+			t.Errorf("[%s] Wrong value for ComplexExtension: got: %s want: %s\n", test.name, ext, &want)
 		}
 	}
 }