mtdparts: fix write through NULL pointer

The "mtdparts add" command wrote through a NULL pointer - on many
systems this went unnoticed (PowerPC has writable RAM there, some ARM
systems have ROM where a write has no effect), but on arm1136
(i.MX31) it crashed the system.

Add appropriate checks.

Signed-off-by: Wolfgang Denk <wd@denx.de>