swscale/utils: Fix potential overflow of dstPos*xInc before converting to 64bit

Fixes CID1061053

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swscale/utils: Fix potential overflow of srcPos*C  before converting to 64bit

Fixes CID1061054

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