cortex_a: remove memory leak on failure

If mem_ap_sel_write_atomic_u32 fails then tmp_buff may not be released.
Detected by clang.

Change-Id: I3d5416bd47d0eea61f61ec02ac5e43c82f114108
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1433
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>