source/sink: initialize sample rate cache variable

Thanks to Jiří Pinkava for bringing this up.

lib/osmosdr_sink_c_impl.cc

@@ -66,7 +66,8 @@ osmosdr_make_sink_c (const std::string &args)
 osmosdr_sink_c_impl::osmosdr_sink_c_impl (const std::string &args)
   : gr_hier_block2 ("osmosdr_sink_c_impl",
         args_to_io_signature(args),
-        gr_make_io_signature (0, 0, 0))
+        gr_make_io_signature (0, 0, 0)),
+    _sample_rate(NAN)
 {
   size_t channel = 0;
   bool device_specified = false;

lib/osmosdr_sink_c_impl.h

@@ -75,6 +75,7 @@ private:
 
   std::vector< osmosdr_snk_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/osmosdr_source_c_impl.cc

@@ -91,7 +91,8 @@ osmosdr_make_source_c (const std::string &args)
 osmosdr_source_c_impl::osmosdr_source_c_impl (const std::string &args)
   : gr_hier_block2 ("osmosdr_source_c_impl",
         gr_make_io_signature (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/osmosdr_source_c_impl.h

@@ -82,6 +82,7 @@ private:
 
   std::vector< osmosdr_src_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;