If the next header frame codes zero envelopes the previous frame's
values will be used. Consequently the invalid values must be cleared.
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
(cherry picked from commit a237b38021)
Signed-off-by: Reinhard Tartler <siretart@tauware.de>
tags/n0.10.3