fix: potential verifier panic in unzip_and_prefix_sums

[erabinov]

The function unzip_and_prefix_sums calls .last().unwrap() on an iterator that is not checked to be non-empty. This PR adds a verifier check before the call to unzip_and_prefix_sums to make sure that the iterator in unzip_and_prefix_sums is non-empty.

[tamirhemo]

LGTM

[github-actions]

Test	Old	New	Diff
bls12_381_tests_test_bls_double_100	6402021	6402021	0.0000 %
curve25519_dalek_test_decompressed_expected_value	4423016	4503209	1.8131 %
k256_test_recover_high_hash_high_recid	1763250	2233713	26.6816 %
p256_test_recover_high_hash_high_recid	6231001	5796453	-6.9740 %
secp256k1_program_test_verify_v0_30_0_rand_lte_100	17073461	17041362	-0.1880 %
k256_test_recover_pubkey_infinity	97611	97611	0.0000 %
bls12_381_tests_test_inverse_fp2_100	2763953	2763953	0.0000 %
p256_test_recover_rand_lte_100	15901743	15915299	0.0852 %
secp256k1_program_test_verify_rand_lte_100	17080665	17085168	0.0264 %
rust_crypto_rsa_test_pkcs_verify_100	28950595	28920515	-0.1039 %
k256_test_schnorr_verify	5676262	5678090	0.0322 %
k256_test_recover_rand_lte_100	4421565	4413237	-0.1883 %
curve25519_dalek_test_zero_mul	71794	71794	0.0000 %
bls12_381_tests_test_sqrt_fp_100	918781	930984	1.3282 %
bls12_381_tests_test_sqrt_fp2_100	1869818	1657574	-11.3511 %
bls12_381_tests_test_bls_add_100	10573008	10573008	0.0000 %
curve25519_dalek_test_add_then_multiply	2732857	2966478	8.5486 %
sha_test_sha3_expected_digest_lte_100_times	1608856	1609012	0.0097 %
bn_test_bn_test_fq_inverse_100	834612	834612	0.0000 %
curve25519_dalek_ng_test_add_then_multiply	4266528	5012912	17.4939 %
sha_test_sha2_v0_10_9_expected_digest_lte_100_times	1765922	1769175	0.1842 %
p256_test_recover_pubkey_infinity	101631	101631	0.0000 %
p256_test_verify_rand_lte_100	11884151	11863144	-0.1768 %
bn_test_bn_test_fr_inverse_100	851612	851612	0.0000 %
curve25519_dalek_ng_test_zero_mul	107777	107777	0.0000 %
rustcrypto_bigint_test_bigint_mul_add_residue	1736233	1736239	0.0003 %
rustcrypto_bigint_test_bigint_mul_mod_special	1753658	1753658	0.0000 %
bn_test_bn_test_g1_double_100	728595	728595	0.0000 %
sha_test_sha2_v0_10_8_expected_digest_lte_100_times	1766023	1768074	0.1161 %
k256_test_verify_rand_lte_100	11880393	11869426	-0.0923 %
curve25519_dalek_ng_test_zero_msm	125373	125373	0.0000 %
bls12_381_tests_test_inverse_fp_100	1434158	1434158	0.0000 %
secp256k1_program_test_recover_v0_30_0_rand_lte_100	5452086	5442958	-0.1674 %
curve25519_dalek_ng_test_decompressed_noncanonical	195589	195589	0.0000 %
curve25519_dalek_test_ed25519_verify	13267400	13271181	0.0285 %
curve25519_dalek_test_zero_msm	83451	83451	0.0000 %
keccack_test_expected_digest_lte_100	1714510	1716098	0.0926 %
secp256k1_program_test_recover_rand_lte_100	5466906	5448859	-0.3301 %
curve25519_dalek_test_decompressed_noncanonical	7659	7659	0.0000 %
sha_test_sha2_v0_10_6_expected_digest_lte_100_times	1766580	1766532	-0.0027 %
bn_test_bn_test_g1_add_100	986109	986109	0.0000 %
sha_test_sha2_v0_9_9_expected_digest_lte_100_times	1259802	1262518	0.2156 %
bn_test_bn_test_fq_sqrt_100	833012	833012	0.0000 %