nrf51: remove dereference of null pointer

found by clang 3.4.

Change-Id: Id499b546f65acd7a719498bc97e33b21d1ba565a
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/2119


Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Andrey Smirnov <andrew.smirnov@gmail.com>