BUG/MINOR: jwt: use CRYPTO_memcmp() to compare HMACs

As Tim reported in github issue #1414, we ought to use a constant-time
memcmp() when comparing hashes to avoid time-based attacks. Let's use
CRYPTO_memcmp() since this code already depends on openssl.

No backport is needed, this was just merged into 2.5.
diff --git a/src/jwt.c b/src/jwt.c
index 0e23305..24459b0 100644
--- a/src/jwt.c
+++ b/src/jwt.c
@@ -205,7 +205,7 @@
 			ctx->jose.length + ctx->claims.length + 1, signature, &signature_length);
 
 	if (hmac_res && signature_length == decoded_signature->data &&
-		  (memcmp(decoded_signature->area, signature, signature_length) == 0))
+		  (CRYPTO_memcmp(decoded_signature->area, signature, signature_length) == 0))
 		retval = JWT_VRFY_OK;
 
 	free_trash_chunk(trash);