libtommath: Fix check mp_init_multi() result

libtommath: Fix check mp_init_multi() result

If the mp_init_multi() call had failed due to memory allocation failure,
mp_div() would have returned 1 instead of MP_MEM (-2). It looks like all
callers are checking the return value against MP_OKAY instead of <1
(etc.), so this does not seem to result in difference in behavior.
Anyway, it's best to fix the mp_div() return value for the MP_MEM error
case to avoid unexpected behavior.

Signed-off-by: Maks Naumov <maksqwe1@ukr.net>