mountmgr: Include the terminating NULL in mount ids.

Some "protection" schemes expect these to be at least 2 bytes long.
diff --git a/dlls/mountmgr.sys/mountmgr.c b/dlls/mountmgr.sys/mountmgr.c
index 73e2554..b9e96dc 100644
--- a/dlls/mountmgr.sys/mountmgr.c
+++ b/dlls/mountmgr.sys/mountmgr.c
@@ -423,7 +423,7 @@
         {
             *p = 'a' + i;
             if (!(link = read_symlink( buffer ))) continue;
-            add_mount_point( driver, DRIVE_FIXED, i, link, strlen(link) );
+            add_mount_point( driver, DRIVE_FIXED, i, link, strlen(link) + 1 );
             RtlFreeHeap( GetProcessHeap(), 0, link );
         }
         RtlFreeHeap( GetProcessHeap(), 0, buffer );