mtdparts: Fix final outstanding issue reported by Coverity

As part of fixing the previously reported issues, it was missed that in
the case of mtdparts_init() we need to make sure that tmp_ep is long
enough to contain PARTITION_MAXLEN and a NULL termination.  Then, to be
sure the buffer is NULL terminated, zero the entire buffer rather than
just ensuring the first character is NULL.

Cc: Lothar Waßmann <LW@KARO-electronics.de>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Reported-by: Coverity (CID: 166329)
Signed-off-by: Tom Rini <trini@konsulko.com>