avc_utils: skip empty NALs from malformed bistreams

Avoid a CHECK and make it the decoder's repsonsibility to handle a
malformed bistream gracefully.

Bug: 34509901
Bug: 33137046
Test: StagefrightTest#testStagefright_bug_27855419_CVE_2016_2463
Change-Id: I2d94f8da63d65a86a9c711c45546e4c695e0f3b4
(cherry picked from commit 91fe76a1)