malloc_simple: calloc: don't call memset if malloc failed

malloc_simple: calloc: don't call memset if malloc failed

malloc_simple() can return 0 if out of memory. Don't call memset
from calloc() in this case but rely on the caller checking
the return value.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>