Validate the surface of snapshot window before draw.

Core will not create surface for the snapshot window if the window was
removed just during launch app sequence, and there won't return any
error message for this case. Validate the out surface before draw to
preventing for crash.

Flag: EXEMPT bugfix
Bug: 351721867
Test: atest StartingSurfaceDrawerTests
Change-Id: I1b7926299b710cf2e965a61c75a362dbd19701f1