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>

libhwcomposer/hwc_uimirror.cpp

@@ -63,6 +63,10 @@ bool UIMirrorOverlay::configure(hwc_context_t *ctx, hwc_layer_1_t *layer)
         // Set overlay state
         ov.setState(sState);
         private_handle_t *hnd = (private_handle_t *)layer->handle;
+        if (!hnd) {
+            ALOGE("%s:NULL private handle for layer!", __FUNCTION__);
+            return false;
+        }
         ovutils::Whf info(hnd->width, hnd->height, hnd->format, hnd->size);
         // Determine the RGB pipe for UI depending on the state
         ovutils::eDest dest = ovutils::OV_PIPE0;