serial_test: fix first write_count_value for next write

src/systemcmds/serial_test/serial_test.c

@@ -598,7 +598,14 @@ static void process_write_data(struct g_mod_t *g_mod, struct cli_args_t *g_cl)
 		count += c;
 
 		if (c <= g_mod->_write_size) {
-			g_mod->_write_count_value = g_mod->_write_data[c];
+
+			if (c == 0) {
+				g_mod->_write_count_value = g_mod->_write_data[0];
+
+			} else {
+				g_mod->_write_count_value = next_count_value(g_mod->_write_data[c - 1], g_cl->_ascii_range);
+			}
+
 			repeat = 0;
 		}
 	} while (repeat);