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: I3cf6e871592826f93b0dcd2b06fff80677bc8338
(cherry picked from commit 5df744afde273bc4d0f7a499581dd2fb2ae6cb45)