xmv: Add more sanity checks for parameters read from the bitstream

xmv: Add more sanity checks for parameters read from the bitstream

Since the number of channels is multiplied by 36 and assigned to
to a uint16_t, make sure this calculation didn't overflow. (In
certain cases the calculation could overflow leaving the
truncated block_align at 0, leading to divisions by zero later.)

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit d4c2a3740fb95f952a87ba320d2bf31f126bdf68)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
(cherry picked from commit 00516b5491fbd99e4057f21eae231fc02cc596e3)