Make the look-back buffer a member variable

lib/m17_decoder_impl.cc

@@ -168,7 +168,6 @@ void m17_decoder_impl::set_debug_ctrl(bool debug)
      int countout=0;
 
      float sample;                       //last raw sample from the stdin
-     float last[8];                      //look-back buffer for finding syncwords
      float xcorr;                        //cross correlation for finding syncwords
      float meanx;                        //mean value
      float normx;                        //cross correlation normalization

lib/m17_decoder_impl.h

@@ -25,6 +25,7 @@ private:
     bool _debug_ctrl=false;
     float _threshold=0.9;
 
+    float last[8] = {0};                //look-back buffer for finding syncwords
     float pld[SYM_PER_PLD];             //raw frame symbols
     uint16_t soft_bit[2*SYM_PER_PLD];   //raw frame soft bits
     uint16_t d_soft_bit[2*SYM_PER_PLD]; //deinterleaved soft bits