target: check memory handlers before use for all types

target: check memory handlers before use for all types

MMU types were checking and installing fakes at init, but this wasn't catching
all devices.  Fixes segfaults when attempting mdw and friends on avr.

Change-Id: I5b11f9913157a21f1aeb11ec852f593b529d9be8
Signed-off-by: Karl Palsson <karlp@tweak.net.au>
Reviewed-on: http://openocd.zylin.com/2791
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Andreas Färber <afaerber@suse.de>