Fix double free.

When a command starting with an asterisk was encountered, ModuleConfig
was called, which calls AddToModList which frees its argument
sometimes. Then __execute_function tries to free the same pointer
again. This commit fixes this by only freeing rline in AddToModList if
it points at xasprintf'd memory, as freeing the memory from xasprintf
won't invalidate expaction.

Fixes #425.

fvwm/modconf.c

@@ -154,7 +154,10 @@ static struct moduleInfoList *AddToModList(char *tline)
 	this->data = expand_vars(rline, NULL, False, True, NULL, exc);
 	strcpy(this->data, rline);
 	exc_destroy_context(exc);
-	free(rline);
+	/* Free rline only if it is xasprintf'd memory (not pointing at tline
+	 * anymore). If we free our tline argument it causes a crash in __execute_function. */
+	if (rline != tline)
+		free(rline);
 
 	this->next = NULL;
 	if(prev == NULL)