Fix applying zero offset to null pointer when creating moduleFreeContextReusedClient (#7323)

Before this fix we where attempting to select a db before creating db the DB, see: #7323

This issue doesn't seem to have any implications, since the selected DB index is 0,
the db pointer remains NULL, and will later be correctly set before using this dummy
client for the first time.

As we know, we call 'moduleInitModulesSystem()' before 'initServer()'. We will allocate
memory for server.db in 'initServer', but we call 'createClient()' that will call 'selectDb()'
in 'moduleInitModulesSystem()', before the databases where created. Instead, we should call
'createClient()' for moduleFreeContextReusedClient after 'initServer()'.

src/module.c

@@ -283,7 +283,7 @@ typedef struct RedisModuleKeyspaceSubscriber {
 static list *moduleKeyspaceSubscribers;
 
 /* Static client recycled for when we need to provide a context with a client
- * in a situation where there is no client to provide. This avoidsallocating
+ * in a situation where there is no client to provide. This avoids allocating
  * a new client per round. For instance this is used in the keyspace
  * notifications, timers and cluster messages callbacks. */
 static client *moduleFreeContextReusedClient;
@@ -7249,6 +7249,16 @@ int moduleRegisterApi(const char *funcname, void *funcptr) {
 /* Global initialization at Redis startup. */
 void moduleRegisterCoreAPI(void);
 
+/* Some steps in module initialization need to be done last after server
+ * initialization.
+ * For example, selectDb() in createClient() requires that server.db has
+ * been initialized, see #7323. */
+void moduleInitModulesSystemLast(void) {
+    moduleFreeContextReusedClient = createClient(NULL);
+    moduleFreeContextReusedClient->flags |= CLIENT_MODULE;
+    moduleFreeContextReusedClient->user = NULL; /* root user. */
+}
+
 void moduleInitModulesSystem(void) {
     moduleUnblockedClients = listCreate();
     server.loadmodule_queue = listCreate();
@@ -7256,9 +7266,6 @@ void moduleInitModulesSystem(void) {
 
     /* Set up the keyspace notification susbscriber list and static client */
     moduleKeyspaceSubscribers = listCreate();
-    moduleFreeContextReusedClient = createClient(NULL);
-    moduleFreeContextReusedClient->flags |= CLIENT_MODULE;
-    moduleFreeContextReusedClient->user = NULL; /* root user. */
 
     /* Set up filter list */
     moduleCommandFilters = listCreate();

src/server.c

@@ -5205,6 +5205,7 @@ int main(int argc, char **argv) {
     #ifdef __linux__
         linuxMemoryWarnings();
     #endif
+        moduleInitModulesSystemLast();
         moduleLoadFromQueue();
         ACLLoadUsersAtStartup();
         InitServerLast();

src/server.h

@@ -1572,6 +1572,7 @@ extern dictType modulesDictType;
 
 /* Modules */
 void moduleInitModulesSystem(void);
+void moduleInitModulesSystemLast(void);
 int moduleLoad(const char *path, void **argv, int argc);
 void moduleLoadFromQueue(void);
 int *moduleGetCommandKeysViaAPI(struct redisCommand *cmd, robj **argv, int argc, int *numkeys);