malloc_simple: fix malloc_ptr calculation

The gd->malloc_ptr and the gd->malloc_limit are offsets to gd->malloc_base.
But the addr variable contains the absolute address. The new_ptr must be:
addr + bytes - gd->malloc_base.

Signed-off-by: Philipp Rosenberger <ilu@linutronix.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>