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 936dcf2)

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from #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)