use nullptr when appropriate

src/zm_stream.cpp

@@ -39,7 +39,7 @@ StreamBase::~StreamBase() {
   closeComms();
   if ( monitor ) {
     delete monitor;
-    monitor = NULL;
+    monitor = nullptr;
   }
 }
 

src/zm_stream.h

@@ -116,7 +116,7 @@ protected:
 public:
   StreamBase(): 
     monitor_id(0),
-    monitor(0),
+    monitor(nullptr),
     type(DEFAULT_TYPE),
     format(""),
     replay_rate(DEFAULT_RATE),