Enforce a strict output length check in CRYPTO_ccm128_tag

Return error if the output tag buffer size doesn't match
the tag size exactly. This prevents the caller from
using that portion of the tag buffer that remains
uninitialized after an otherwise succesfull call to
CRYPTO_ccm128_tag.

Bug found by OSS-Fuzz.

Fix suggested by Kurt Roeckx.

Signed-off-by: Guido Vranken <guidovranken@gmail.com>

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8810)

crypto/modes/ccm128.c

@@ -425,7 +425,7 @@ size_t CRYPTO_ccm128_tag(CCM128_CONTEXT *ctx, unsigned char *tag, size_t len)
 
     M *= 2;
     M += 2;
-    if (len < M)
+    if (len != M)
         return 0;
     memcpy(tag, ctx->cmac.c, M);
     return M;