correct stat collection for mixer frame time usage

audio-mixer/src/main.cpp

@@ -412,6 +412,9 @@ int main(int argc, const char* argv[]) {
             
             if (percentageOfMaxElapsed > 0) {
                 sumFrameTimePercentages += percentageOfMaxElapsed;
+            } else {
+                // a negative value suggests that we've taken 100% of the allowable time
+                sumFrameTimePercentages += 100;
             }
             
             numStatCollections++;