DO NOT MERGE: Decoder: Conceal picture only if valid picture buffer is obtained.

If all the slices in the current pic were invalid, then
the decoder would not have received a valid picture buffer
in the current call. In such cases there is no need to conceal or
deblock the picture.

Bug: 62896384
Test: run ASAN-enabled PoC before/after the patch
Change-Id: Ia7d979a78ae3f4fb443a1759c8e6cf8780565ad8
Merged-In: I3cf6e871592826f93b0dcd2b06fff80677bc8338