Do not copy the memory when the buffer is empty

This fixes the following undefined behaviour

converter-example.c:721:13: runtime error: null pointer passed as argument 2, which is declared to never be null
/usr/include/string.h:44:28: note: nonnull attribute specified here

skeletons/converter-example.c

@@ -717,9 +717,11 @@ static void add_bytes_to_buffer(const void *data2add, size_t bytes) {
             perror("malloc()");
             exit(EX_OSERR);
         }
+		if (DynamicBuffer.length) {
 			memcpy(p,
 					DynamicBuffer.data + DynamicBuffer.offset,
 					DynamicBuffer.length);
+		}
         FREEMEM(DynamicBuffer.data);
         DynamicBuffer.data = (uint8_t *)p;
         DynamicBuffer.offset = 0;