Add timestamp snprintf. Fix bacapp snprintf to account for string size zero behavior of snprintf.

src/bacnet/timestamp.h

@@ -113,6 +113,11 @@ extern "C" {
     bool bacapp_timestamp_init_ascii(
         BACNET_TIMESTAMP *timestamp,
         const char *ascii);
+    BACNET_STACK_EXPORT
+    int bacapp_timestamp_to_ascii(
+        char *str, 
+        size_t str_size, 
+        BACNET_TIMESTAMP *timestamp);
 
 #ifdef __cplusplus
 }