hwc: more error checks

-- numHwLayers==1 means that there is only the FB_TARGET which HWC shouldn't
   draw. Also check for blank in set.  Checking for this in prepare prevents us
   from acquiring pipes.
-- add null check for private handle
-- print out vsync timestamp data on error

bug: 7274417
related-to-bug: 7288769
Change-Id: I8dae0d0748c5226fdc33ed8f5cc5259835dec2de
Signed-off-by: Iliyan Malchev <malchev@google.com>

libgralloc/framebuffer.cpp

@@ -113,7 +113,7 @@ static int fb_post(struct framebuffer_device_t* dev, buffer_handle_t buffer)
         m->info.yoffset = hnd->offset / m->finfo.line_length;
         m->commit.var = m->info;
         if (ioctl(m->framebuffer->fd, MSMFB_DISPLAY_COMMIT, &m->commit) == -1) {
-            ALOGE("%s: FBIOPUT_VSCREENINFO failed for external, err: %s", __FUNCTION__,
+            ALOGE("%s: MSMFB_DISPLAY_COMMIT ioctl failed, err: %s", __FUNCTION__,
                     strerror(errno));
             return -errno;
         }