submodule: Do not try to update a missing submodule

If a submodule has been configured but not yet added, do not try to update it.

Issue #6433: git_submodule_update fails to update configured but missing submodule

src/libgit2/submodule.c

@@ -1338,7 +1338,11 @@ int git_submodule_update(git_submodule *sm, int init, git_submodule_update_optio
 	/* Get the status of the submodule to determine if it is already initialized  */
 	if ((error = git_submodule_status(&submodule_status, sm->repo, sm->name, GIT_SUBMODULE_IGNORE_UNSPECIFIED)) < 0)
 		goto done;
-
+	
+	/* If the submodule is configured but hasn't been added, skip it */
+	if (submodule_status == GIT_SUBMODULE_STATUS_IN_CONFIG)
+	        goto done;
+	
 	/*
 	 * If submodule work dir is not already initialized, check to see
 	 * what we need to do (initialize, clone, return error...)