Use isNaN instead of comparing to NaN

[lotodore]

When compiling the script with the google closure tools, the following warnings occur:

inc\third_party\protobuf.js:1175: WARNING - Comparison again NaN is always false. Did you mean isNaN()?
if (n == NaN) {
^

inc\third_party\protobuf.js:1188: WARNING - Comparison again NaN is always false. Did you mean isNaN()?
if (n == NaN) {
^

inc\third_party\protobuf.js:1339: WARNING - Comparison again NaN is always false. Did you mean isNaN()?
if (n == NaN) {
^

According to the compiler, isNaN should be used in this code.