gralloc4-vts: don't wait on invalid sync fence

Sync fences can be invalid if the buffer is already ready. If the
fence is invalid, we do not need to wait.

Test: VtsHalGraphicsMapperV4_0
Bug: 150213134

Change-Id: I8326d5f8b358c466ee9a686fd299daed1d4b4aa8
1 file changed