misc: update: fix buffer overflow in updater

On 32 bit builds, parsing of update status files with a size of
4294967295 or more lead to an integer truncation in a call to malloc
and a subsequent buffer overflow. This happened prior to checking the
files' signature. The commit fixes this by disallowing overly large
status files (above 65k in practice)

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