gdb server: do not free tdesc_filename early

Issue caught by clang.

Change-Id: I4fb331574512140b36e56b3b7ab7e8bdad5e013d
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1554
Tested-by: jenkins