Add snapshot nullptr check in SF#renderScreenImpl

A layer that is not parented to the layer hierarchy could
have a null snapshot, in which case we can ignore it when
finding the display.

Fixes: b/331874495, b/331923495
Test: presubmit
Change-Id: I06ed68d2af4afd47fb83f8c38ba11b70fe6700f0
1 file changed