fix [2170283] SurfaceFlinger crashes on OOM.

when running out of memory, a null handle is returned but the error code may not be set.
In that case we need to return NO_MEMORY instead of NO_ERROR, so that the calling code
won't try to dereference the null pointer.

libs/ui/Surface.cpp

@@ -746,6 +746,8 @@ status_t Surface::getBufferLocked(int index, int usage)
                 currentBuffer->setIndex(index);
                 mNeedFullUpdate = true;
             }
+        } else {
+            err = err<0 ? err : NO_MEMORY;
         }
     }
     return err;