uboot-phytec

cmd: mtdparts: fix mtdparts variable presence confusion in mtdparts_init

A private buffer is used to read mtdparts variable from non-relocated
environment. A pointer to that buffer is returned unconditionally,
confusing later test for variable presence in the environment.
Fix it by returning NULL when getenv_f fails.

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