cmd: mtdparts: fix uninitialized variable warning

commit 06a040a3 ("cmd: mtdparts: fix null pointer dereference in parse_mtdparts")
removed the initialization of a pointer variable, which is
subsequently used in a debug() call. This produces an uninitialized
variable warning, when compiling with DEBUG defined.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>