gdb_server: Don't modify "buf" argument in decode_xfer_read()

Make a temporary copy of argument "buf" before modifying it.
This requires also returning annex as copy of "buf".

This change is necessary in order to make packet[] "const".

Change-Id: I41eddc2edba1a88384aa7f5591fe50f6ee6a135c
Signed-off-by: Christian Eggers <ceggers@gmx.de>
---
Changelog:
v4:
- Initialize annex to NULL
v3:
- Return "annex" as copy instead of removing it.
Reviewed-on: http://openocd.zylin.com/1924


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>