xrow: get rid of unsafe cast of unsigned to signed

Casting a uint64_t greater than INT64_MAX to int64_t is
implementation-defined behaviour, according to the C standard. Let's
avoid that.

In both cases fixed `len` is uint32_t and `ibuf_used(in)` returns a
size_t (aka uint64_t on every platform that we care about).
Hence the result of the subtraction is uint64_t and better use
it directly. Besides, `coio_breadn_timeout` also takes a size_t.

While I'm at it, let's actually change `len` to be uint64_t:
`mp_decode_uint()` returns that anyway.

Closes tarantool/security#108
Closes tarantool/security#109

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring