cafdec: fix overflow checking in read_header()

Several compilers such as clang/icc/pathscale will optimize the check
pos + size < pos (assuming size > 0) into false, since signed integer
overflow is undefined behavior in C.  This breaks overflow checking.
Use a safe precondition check instead.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>