gdb_server: Fix segfault in (and rewrite) decode_xfer_read

Introduced by 537b06a8 (free non-malloced memory).

Rewrite to use standard C string routines and make returning annex
optional since it's not currently used.

Change-Id: Idf3698a482dfeff7fa5ea1660fd89122eb80b68d
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/2023


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>