[wpimath] LinearFilter lastValue(): Fix runtime exception (#6713)

wpimath/src/main/java/edu/wpi/first/math/filter/LinearFilter.java

@@ -56,6 +56,7 @@ public class LinearFilter {
   private final DoubleCircularBuffer m_outputs;
   private final double[] m_inputGains;
   private final double[] m_outputGains;
+  private double m_lastOutput;
 
   private static int instances;
 
@@ -310,6 +311,7 @@ public class LinearFilter {
       m_outputs.addFirst(retVal);
     }
 
+    m_lastOutput = retVal;
     return retVal;
   }
 
@@ -319,7 +321,7 @@ public class LinearFilter {
    * @return The last value.
    */
   public double lastValue() {
-    return m_outputs.getFirst();
+    return m_lastOutput;
   }
 
   /**