Prevent a crash when opening contextual subtables

I don't know why it is crshing now (may be the resources?) as this code have
been like that since the dawn of time.

Traceback:

  0xb78289b0 in check_image_buffers (gdisp=0x84d5c78, neww=<optimized out>, newh=7, is_bitmap=0) at gimagexdraw.c:1705
  1705	    if ( width > gdisp->gg.iwidth || depth!=gdisp->gg.img->depth ) {
  (gdb) bt
  #0  0xb78289b0 in check_image_buffers (gdisp=0x84d5c78, neww=<optimized out>, newh=7, is_bitmap=0) at gimagexdraw.c:1705
  #1  0xb782b595 in gximage_to_ximage (image=0x8527ff0, src=0xbfffa0a0, gw=<optimized out>) at gimagexdraw.c:1778
  #2  0xb782e8c2 in _GXDraw_Image (_w=0x9bf4bb0, image=<optimized out>, src=0xbfffa0a0, x=8250, y=7) at gimagexdraw.c:1981
  #3  0xb7807a98 in GDrawDrawScaledImage (w=0x9bf4bb0, img=0x8527ff0, x=8250, y=7) at gdraw.c:488
  #4  0xb781b513 in GListMarkDraw (pixmap=0x9bf4bb0, x=8250, y=1, height=19, state=gs_enabled) at ggadgets.c:482
  #5  0xb788c493 in GMatrixEdit_SubExpose (gme=0x9bd9448, pixmap=0x9bf4bb0, event=0xbfffa400) at gmatrixedit.c:1691
  #6  0xb788c6b1 in matrixeditsub_e_h (gw=0x9bf4bb0, event=0xbfffa400) at gmatrixedit.c:1723
  #7  0xb78040da in _GWidget_Container_eh (gw=0x9bd9178, event=0xbfffa400) at gcontainer.c:269
  #8  0xb7878001 in dispatchEvent (gdisp=0x84d5c78, event=0xbfffa5ec) at gxdraw.c:3959
  #9  0xb7878162 in GXDrawProcessOneEvent (gdisp=0x84d5c78) at gxdraw.c:3991
  #10 0xb7808606 in GDrawProcessOneEvent (gdisp=0x84d5c78) at gdraw.c:748
  #11 0x080c77ff in ContextChainEdit (sf=0x9aa3c68, fpst=0x8ad982c, gfi=0x9afbfc0, newname=0x0, layer=1) at contextchain.c:3207
  #12 0x0818f14a in _LookupSubtableContents (sf=0x9aa3c68, sub=0x8a9d43c, sd=0x0, def_layer=1) at lookupui.c:5531
  #13 0x0813138d in LookupSubtableContents (gfi=0x9afbfc0, isgpos=0) at fontinfo.c:5941
  #14 0x08136a16 in LookupMouse (gfi=0x9afbfc0, isgpos=0, event=0xbfffe930) at fontinfo.c:7336
  #15 0x08136ccc in lookups_e_h (gw=0x9b98230, event=0xbfffe930, isgpos=0) at fontinfo.c:7380
  #16 0x08136d78 in gsublookups_e_h (gw=0x9b98230, event=0xbfffe930) at fontinfo.c:7399
  #17 0xb788f3b0 in drawable_e_h (pixmap=0x9b98230, event=0xbfffe930) at gdrawable.c:219
  #18 0xb78048a7 in _GWidget_Container_eh (gw=0x9b98230, event=0xbfffe930) at gcontainer.c:393
  #19 0xb7878001 in dispatchEvent (gdisp=0x84d5c78, event=0xbfffeb1c) at gxdraw.c:3959
  #20 0xb7878480 in GXDrawEventLoop (gd=0x84d5c78) at gxdraw.c:4058
  #21 0xb78086c1 in GDrawEventLoop (gdisp=0x84d5c78) at gdraw.c:766
  #22 0x0822457c in main (argc=1, argv=0xbffff134) at startui.c:1501

gdraw/gimagexdraw.c

@@ -1702,7 +1702,7 @@ static void check_image_buffers(GXDisplay *gdisp, int neww, int newh, int is_bit
 	width = neww;
 	if ( width<400 ) width = 400;
     }
-    if ( width > gdisp->gg.iwidth || depth!=gdisp->gg.img->depth ) {
+    if ( width > gdisp->gg.iwidth || (gdisp->gg.img!=NULL && depth!=gdisp->gg.img->depth) ) {
 	if ( depth<=8 ) {
 	    if ( gdisp->gg.red_dith!=NULL ) free(gdisp->gg.red_dith);
 	    if ( gdisp->gg.green_dith!=NULL ) free(gdisp->gg.green_dith);