Fix OOB read in EC_GROUP_new_from_params() with zero-length generator

Fix OOB read in EC_GROUP_new_from_params() with zero-length generator

When OSSL_PKEY_PARAM_EC_GENERATOR is provided as an octet string of
length 0, buf[0] is read before validating data_size, causing a
heap-buffer-overflow detectable under ASan.

Reject zero-length generator octet strings before the dereference.

CLA: trivial
Resolves: https://github.com/openssl/openssl/issues/31125
Fixes: c0f39ded68ba "Add Explicit EC parameter support to providers."
Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Frederik Wedel-Heinen <fwh.openssl@gmail.com>
MergeDate: Mon May 11 08:34:15 2026
(Merged from https://github.com/openssl/openssl/pull/31128)