string_output_get_string() uses g_string_free(str, false) to
transfer the 'str' pointer to callers and never free it.

Signed-off-by: Chen Fan <chen.fan.fnst@cn.fujitsu.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Hu Tao <hutao@cn.fujitsu.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>