TLS1.3: Correct intermediate secret derivation

This label for this derivation was incorrectly "derived" or "der" depending
on the pointer size of the build(!). The correct string is "derived secret".

(cherry picked from commit 936dcf272033c1bf59a5e859ec63e2557194f191)
Reviewed-by: NTim Hudson <tjh@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2989)

ssl/tls13_enc.c

@@ -124,7 +124,7 @@ int tls13_generate_secret(SSL *s, const EVP_MD *md,
     size_t mdlen, prevsecretlen;
     int ret;
     EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
-    const char *derived_secret_label = "derived secret";
+    static const char derived_secret_label[] = "derived secret";
     unsigned char preextractsec[EVP_MAX_MD_SIZE];
 
     if (pctx == NULL)