source/sink: initialize sample rate cache variable

Thanks to Jiří Pinkava for bringing this up.

lib/sink_impl.cc

@@ -65,7 +65,8 @@ osmosdr::sink::make( const std::string &args )
 sink_impl::sink_impl( const std::string &args )
   : gr::hier_block2 ("sink_impl",
         args_to_io_signature(args),
-        gr::io_signature::make(0, 0, 0))
+        gr::io_signature::make(0, 0, 0)),
+    _sample_rate(NAN)
 {
   size_t channel = 0;
   bool device_specified = false;

lib/sink_impl.h

@@ -71,6 +71,7 @@ public:
 private:
   std::vector< sink_iface * > _devs;
 
+  /* cache to prevent multiple device calls with the same value coming from grc */
   double _sample_rate;
   std::map< size_t, double > _center_freq;
   std::map< size_t, double > _freq_corr;

lib/source_impl.cc

@@ -91,7 +91,8 @@ osmosdr::source::make( const std::string &args )
 source_impl::source_impl( const std::string &args )
   : gr::hier_block2 ("source_impl",
         gr::io_signature::make(0, 0, 0),
-        args_to_io_signature(args))
+        args_to_io_signature(args)),
+    _sample_rate(NAN)
 {
   size_t channel = 0;
   bool device_specified = false;

lib/source_impl.h

@@ -78,6 +78,7 @@ public:
 private:
   std::vector< source_iface * > _devs;
 
+  /* cache to prevent multiple device calls with the same value coming from grc */
   double _sample_rate;
   std::map< size_t, double > _center_freq;
   std::map< size_t, double > _freq_corr;