fixup Valgrind warning:

Conditional jump or move depends on uninitialised value(s)
strncpy (vg_replace_strmem.c:599)
MIT_SetCurBackground (m_menu.c:2650)
M_IterateMenuTree (m_menu.c:2581)
M_SetMenuCurBackground (m_menu.c:2790)
F_InitMenuPresValues (f_finale.c:2276)
D_StartTitle (d_main.c:998)
G_Responder (g_game.c:2117)
D_ProcessEvents (d_main.c:282)
Local_Maketic (d_clisrv.c:5297)
NetUpdate (d_clisrv.c:5701)
TryRunTics (d_clisrv.c:5371)
D_SRB2Loop (d_main.c:820)