Fixed crash in BitBlt when source DC is NULL.

graphics/bitblt.c

@@ -85,7 +85,7 @@ BOOL WINAPI BitBlt( HDC hdcDst, INT xDst, INT yDst, INT width,
               hdcDst, xDst, yDst, width, height, dcDst->bitsPerPixel, rop);
         if (dcDst->funcs->pBitBlt)
             ret = dcDst->funcs->pBitBlt( dcDst->physDev, xDst, yDst, width, height,
-                                         dcSrc->physDev, xSrc, ySrc, rop );
+                                         dcSrc ? dcSrc->physDev : NULL, xSrc, ySrc, rop );
         if (dcSrc) GDI_ReleaseObj( hdcSrc );
         GDI_ReleaseObj( hdcDst );
     }

graphics/x11drv/bitblt.c

@@ -1503,7 +1503,7 @@ BOOL X11DRV_BitBlt( X11DRV_PDEVICE *physDevDst, INT xDst, INT yDst,
     BOOL result = FALSE;
     INT sSrc, sDst;
     RECT visRectDst, visRectSrc;
-    DC *dcSrc = physDevSrc->dc;
+    DC *dcSrc = physDevSrc ? physDevSrc->dc : NULL;
     DC *dcDst = physDevDst->dc;
 
     if (((rop >> 16) & 0x55) == ((rop >> 17) & 0x55)) {