Xephyr: Handle source-only pictures in ephyrPrepare/DoneComposite

There is no pixmap associated with source-only pictures.

Fixes Xephyr -fakexa crashing on startup.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

hw/kdrive/ephyr/ephyr_draw.c

@@ -254,7 +254,8 @@ ephyrPrepareComposite(int op, PicturePtr pSrcPicture, PicturePtr pMaskPicture,
     EphyrFakexaPriv *fakexa = scrpriv->fakexa;
 
     ephyrPreparePipelinedAccess(pDst, EXA_PREPARE_DEST);
-    ephyrPreparePipelinedAccess(pSrc, EXA_PREPARE_SRC);
+    if (pSrc != NULL)
+        ephyrPreparePipelinedAccess(pSrc, EXA_PREPARE_SRC);
     if (pMask != NULL)
         ephyrPreparePipelinedAccess(pMask, EXA_PREPARE_MASK);
 
@@ -298,7 +299,8 @@ ephyrDoneComposite(PixmapPtr pDst)
 
     if (fakexa->pMask != NULL)
         ephyrFinishPipelinedAccess(fakexa->pMask, EXA_PREPARE_MASK);
-    ephyrFinishPipelinedAccess(fakexa->pSrc, EXA_PREPARE_SRC);
+    if (fakexa->pSrc != NULL)
+        ephyrFinishPipelinedAccess(fakexa->pSrc, EXA_PREPARE_SRC);
     ephyrFinishPipelinedAccess(fakexa->pDst, EXA_PREPARE_DEST);
 }