A negative `size' will bypass FFMIN().  In the subsequent memcpy() call,
`size' will be considered as a large positive value, leading to a buffer
overflow.

Change the type of `size' to unsigned int to avoid buffer overflow, and
simplify overflow checks accordingly.
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>