egl-wayland: Handle failure to acquire image in wlEglSendDamageEvent

The image parameter of send_explicit_sync_points is assumed not to be
null, however this is a case the rest of the code handles. This causes
sporadic problems on KDE when running overnight as the image will not
be valid at some point, causing us to crash.

Fixes #143

src/wayland-eglsurface.c

@@ -265,11 +265,13 @@ wlEglSendDamageEvent(WlEglSurface *surface,
         }
 
         image = pop_acquired_image(surface);
-        if (image) {
-            surface->ctx.currentBuffer = image->buffer;
-            image->attached = EGL_TRUE;
+        if (!image) {
+            return EGL_FALSE;
         }
 
+        surface->ctx.currentBuffer = image->buffer;
+        image->attached = EGL_TRUE;
+
         /*
          * Send our explicit sync acquire and release points. This needs to be done
          * as part of the surface attach as it is a protocol error to specify these