target: fix registers reading from non examined target

target: fix registers reading from non examined target

If a target is not examined when the debugger tries to connect to it
then it can lead to undesired/undefined behavior.

In particular it leads to a zero pointer dereference on the aarch64.

Change-Id: I67f2b714ab8b2727fd36f3de16d7f9017b4c55fe
Signed-off-by: Mikhail Rasputin <mikhail.godlike.rasputin@yandex.ru>
Reviewed-on: http://openocd.zylin.com/5727
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>