mpc8: check stream count before accessing stream 1.

Fixes null pointer dereference

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lcldec: Check length before unsigned subtraction.

Fix integer overflow and out of array read

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
gifdec: check that the last keyframe exists and has been successfully parsed.

Prevents inconsistent state and null pointer dereference

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
mpeg4videoenc: check w,h to be within the supported range.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>