cortex_a/r/m: fix handling of un-examined cores

On multi-core systems, with some cores in power-down state, examination
will fail for these cores. Make sure assert- and deassert_reset functions
don't crash due to uninitialized variables.

Change-Id: I472f8d19af2cd3c770c05f3e57a31b35a863b687
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/3552


Tested-by: jenkins
Reviewed-by: Jiri Kastner <cz172638@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>