gmac: clear the cached message on init - #474

Problem

wp_gmac_init() never cleared the cached message buffer. wolfSSL has no
streaming GMAC API, so wp_gmac_update() accumulates the whole message in
macCtx->data / macCtx->dataLen and wp_gmac_final() feeds it to the
one-shot wc_GmacUpdate(); only wp_gmac_free() and wp_gmac_dup() ever
touched that buffer. Re-initializing a used EVP_MAC_CTX — the documented way
to reuse one, and the natural GMAC pattern since every message needs a fresh IV
— therefore authenticated previous_message || new_message, reporting success
at every call. A wrong tag with no error.

OpenSSL's default provider resets on every init path (gmac_init() always
calls EVP_EncryptInit_ex()), and OpenSSL's evp_test re-inits and re-runs
every MAC KAT, so scripts/test-openssl.sh reproduces this today: all nine
GMAC entries in scripts/evp_test/evpmac_common.txt fail with TEST_MAC_ERR.

Same defect class as f_11550 (CMAC, PR #473); this is its GMAC sibling.

Fix (src/wp_gmac.c)

wp_gmac_init() discards the cached message once the running check passes and
before the parameters and key are applied, so every init starts a new message:

• OPENSSL_clear_free() cleanses the plaintext and releases the
  allocation, keeping dataLen equal to the allocated size — the invariant
  wp_gmac_free() and wp_gmac_dup() both rely on for a correct cleanse and
  memdup.
• data = NULL; dataLen = 0 so wp_gmac_update()'s next
  OPENSSL_realloc() starts clean.

Clearing in init covers both a reset after final and a reset mid-stream, so
wp_gmac_final() is unchanged. No re-keying is needed — wc_GmacUpdate() is a
one-shot over an already-keyed Gmac.

Closes f_11551.

Tests

test_gmac_reinit drives one EVP_MAC_CTX through init/update/final rounds
and an init mid-stream, re-supplying the IV on each init as evp_test does,
and compares every MAC against OpenSSL.

Verification

• Build clean under -Werror.
• Negative control fails before the fix — GMAC after reset doesn't match the first MAC — while the OpenSSL half of the same test passes.
• 212/212 unit tests pass; ASan + UBSan clean.
• evp_test on evpmac_common.txt: 11 errors → 2. All nine GMAC failures
  clear; the two remaining CMAC failures are PR cmac: take a NULL key in init as a restart with the cached key #473 .