Fix fips cleanup of master key

With the conversion to a fixed array, we can no longer just clean the
top level sparse array, as we don't enjoy the NULL check within the
sa_doall loop.

Add a NULL check in the fips cleanup routine to make up for this

Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27794)

crypto/threads_common.c

@@ -403,7 +403,8 @@ void CRYPTO_THREAD_clean_local_for_fips(void)
         return;
 
     mkey = CRYPTO_THREAD_get_local(&master_key);
-    clean_master_key(mkey);
+    if (mkey != NULL)
+        clean_master_key(mkey);
     CRYPTO_THREAD_cleanup_local(&master_key);
 }
 #endif