fix crash on exit

Currently shows
$ ./linux/linuxxdoom
                            DOOM Shareware Startup v1.10
V_Init: allocate screens.
M_LoadDefaults: Load system defaults.
Z_Init: Init zone memory allocation daemon.
W_Init: Init WADfiles.
 adding ./doom1.wad
===========================================================================
                                Shareware!
===========================================================================
M_Init: Init miscellaneous info.
R_Init: Init DOOM refresh daemon - [..                 ]
InitTextures
InitFlats........
InitSprites
InitColormaps
R_InitData
R_InitPointToAngle
R_InitTables
R_InitPlanes
R_InitLightTables
R_InitSkyMap
R_InitTranslationsTables
P_Init: Init Playloop state.
I_Init: Setting up machine state.
Could not start sound server [sndserver]
D_CheckNetGame: Checking network game status.
startskill 2  deathmatch: 0  startmap: 1  startepisode: 1
player 1 of 1 (1 nodes)
S_Init: Setting up sound.
S_Init: default sfx volume 8
HU_Init: Setting up heads up display.
ST_Init: Init status bar.
Error: xdoom currently only supports 256-color PseudoColor screens

doom/linuxdoom-1.10/i_video.c

@@ -172,7 +172,9 @@ void I_ShutdownGraphics(void)
   shmctl(X_shminfo.shmid, IPC_RMID, 0);
 
   // Paranoia.
-  image->data = NULL;
+  if (image){
+  	image->data = NULL;
+  }
 }