Fix incorrect iteration type in extension_file_exists()

Commit f3c9e341cd changed the type of objects in the List that
get_extension_control_directories() returns, from "char *" to
"ExtensionLocation *", but missed adjusting this one caller.

Author: Chao Li <lic@highgo.com>
Discussion: https://www.postgresql.org/message-id/362EA9B3-589B-475A-A16E-F10C30426E28@gmail.com

src/backend/commands/extension.c

@@ -2686,9 +2686,9 @@ extension_file_exists(const char *extensionName)
 
 	locations = get_extension_control_directories();
 
-	foreach_ptr(char, location, locations)
+	foreach_ptr(ExtensionLocation, location, locations)
 	{
-		dir = AllocateDir(location);
+		dir = AllocateDir(location->loc);
 
 		/*
 		 * If the control directory doesn't exist, we want to silently return
@@ -2700,7 +2700,7 @@ extension_file_exists(const char *extensionName)
 		}
 		else
 		{
-			while ((de = ReadDir(dir, location)) != NULL)
+			while ((de = ReadDir(dir, location->loc)) != NULL)
 			{
 				char	   *extname;