sh: Set gd->malloc_base if MALLOC_F_LEN is set

The gd->malloc_base must be set before the C runtime if the MALLOC_F_LEN
is non-zero, otherwise we hit assertion in dlmalloc.c initf_malloc(). So
set it.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>