Modules: fix thread safe context creation crash.

See #6525, this likely creates a NULL deference if the client was
terminated by Redis between the creation of the blocked client and the
creation of the thread safe context.

src/module.c

@@ -4463,7 +4463,7 @@ RedisModuleCtx *RM_GetThreadSafeContext(RedisModuleBlockedClient *bc) {
     ctx->client = createClient(NULL);
     if (bc) {
         selectDb(ctx->client,bc->dbid);
-        ctx->client->id = bc->client->id;
+        if (bc->client) ctx->client->id = bc->client->id;
     }
     return ctx;
 }