Improve scratch usage in ecc_mod_inv.

Improve scratch usage in ecc_mod_inv.

* ecc-mod-inv.c (ecc_mod_inv): Use passed in scratch for all
scratch needs, don't use memory after the result area.
* ecc-ecdsa-sign.c (ecc_ecdsa_sign): Update invert call.
* ecc-ecdsa-verify.c (ecc_ecdsa_verify): Likewise.
* ecc-eh-to-a.c (ecc_eh_to_a): Likewise.
* ecc-j-to-a.c (ecc_j_to_a): Likewise.
* ecc-gostdsa-verify.c (ecc_gostdsa_verify): Likewise.
* ecc-internal.h (ECC_MOD_INV_ITCH, ECC_J_TO_A_ITCH)
(ECC_EH_TO_A_ITCH): Update accordingly, but no change in total
scratch need.