cmd: mtdparts: fix null pointer dereference in parse_mtdparts

In case there is no mtdparts variable in relocated environment,
NULL is assigned to p, which is later fed to strncpy.
Also function parameter mtdparts is completely ignored, so use it
in case mtdparts variable is not found in environment. This
parameter is checked not to be NULL in caller.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>