Mp4: fixed setting wrong mdat atom size in very rare cases. Atom size is the sum of atom header size and atom data size. The specification says that the first 4 bytes are set to one when the atom size is greater than the maximum unsigned 32-bit value. Which means atom header size should be considered when the comparison takes place between atom data size and 0xffffffff.

# Copyright (C) Igor Sysoev
# Copyright (C) Nginx, Inc.


have=NGX_SUPPRESS_WARN . auto/have

have=NGX_SMP . auto/have