sandbox: ensure that state->ram_buf is in low memory

Addresses in state->ram_buf must be in the low 4 GiB of the address space.
Otherwise we cannot correctly fill SMBIOS tables. This shows up in warnings
like:

    WARNING: SMBIOS table_address overflow 7f752735e020

Ensure that state->ram_buf is initialized by the first invocation of
os_malloc().

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>