update keylist to use bool and stdint value for key not found. (#572)

* Updated keylist to use bool and stdint value for key not found.

* Added Keylist_Index_Key to deprecate Keylist_Key function

* Changed the deprecated Keylist_Key API to Keylist_Index_Key

* Added unit test for Keylist_Index_Key API

src/bacnet/basic/sys/keylist.h

@@ -25,6 +25,7 @@
 #define KEYLIST_H
 
 #include "bacnet/bacnet_stack_exports.h"
+#include "bacnet/basic/sys/platform.h"
 #include "bacnet/basic/sys/key.h"
 
 /* This is a key sorted linked list data library that */
@@ -104,12 +105,20 @@ extern "C" {
         OS_Keylist list,
         int index);
 
-/* return the key at the given index */
+/* returns the node key specified by the index */
+    BACNET_STACK_DEPRECATED("Use Keylist_Index_Key() instead")
     BACNET_STACK_EXPORT
     KEY Keylist_Key(
         OS_Keylist list,
         int index);
 
+/* returns the node key specified by the index */
+    BACNET_STACK_EXPORT
+    bool Keylist_Index_Key(
+        OS_Keylist list, 
+        int index, 
+        KEY *pKey);
+
 /* returns the next empty key from the list */
     BACNET_STACK_EXPORT
     KEY Keylist_Next_Empty_Key(