Fix xwayland related crash when mousing out of a window

XWayland likes to set a buffer on the cursor surface then delete it before
we release it.  I'm pretty sure when a client does that we're within spec
to just kill it, but users will likely find this response ungratifying.

So, instead, just gracefully fail to render the undefined surface.

@ref T5593

src/bin/e_pixmap.c

@@ -897,6 +897,11 @@ e_pixmap_image_refresh(E_Pixmap *cp)
 
            if (cp->held_buffer) _e_pixmap_wayland_image_clear(cp);
 
+           /* This catches the case where a client (*cough* xwayland)
+            * deletes a buffer we haven't released
+            */
+           if (!cp->buffer) return EINA_FALSE;
+
            if (!cp->buffer->shm_buffer) return EINA_TRUE;
 
            cp->held_buffer = cp->buffer;