codec: schroedinger: fix potential buffer overflow.

The variable len is a raw 32 bit value read using GetDWBE. If this
value is larger than UINT32_MAX - sizeof(eos), this will cause an
integer overflow in the subsequent call to malloc, and finally a
buffer overflow when calling memcpy. We fix this by checking len
accordingly.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>