AM demod: fill AGC average with ones when passing squelch threshold to avoid abrupt start at squelch opening

plugins/channelrx/demodam/amdemod.h

@@ -160,6 +160,10 @@ private:
         {
             if (m_squelchCount <= m_running.m_audioSampleRate / 10)
             {
+                if (m_squelchCount == m_running.m_audioSampleRate / 20) {
+                    m_volumeAGC.fill(1.0);
+                }
+
                 m_squelchCount++;
             }
         }

sdrbase/dsp/agc.h

@@ -85,6 +85,11 @@ public:
 			m_moving_average.resize(historySize, initial);
 	}
 
+	void fill(double value)
+	{
+	    m_moving_average.fill(value);
+	}
+
 	Real getValue()
 	{
 			if (m_moving_average.average() > m_clip)