Fix segfault (coredump) if get_var_data is NULL

src/util.c

@@ -107,6 +107,10 @@ char *parse_vars(char *string)
             syn_error("ss:error:variable syntax error!");
         
         variable_pointer = get_var_data(varbuffer);
+
+        if(variable_pointer == NULL)
+            syn_error("ss:error:variable syntax error!");
+
         var_len = strlen(variable_pointer);
         buf_len = strlen(finished) + var_len;