armv4_5: prevent segfault when gdb connects to an underinitialised target

This prevents segmentation fault that can occur on cortex_a targets if
gdb connection is established before it's halted and CPSR examined.

Change-Id: Id996200e0fd95440496509c1fecaabbdbf425e23
Tested-by: Henrik Nordstrom <hno@squid-cache.org>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1446
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>