gdb_server: Further cleanup of target desc functions

gdb_server: Further cleanup of target desc functions

Fix use of uninitialized pointer passed to xml_printf, caught by valgrind.

Make sure a failed gdb_generate_target_description frees all allocated
memory and avoids touching its out argument.

Plug memory leak and check allocation in handle_gdb_save_tdesc_command.

Change-Id: I30e20f6760a6215b1b4496304acdf47347eed829
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1645
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Franck Jullien <franck.jullien@gmail.com>