box: refactor port_c msgpack methods

Methods `get_msgpack` and `dump_msgpack` generally do the same things
except for some details (array header, type of destination buffer), so
let's factor out their core part into a separate function - it doesn't
make much sense right now, but we are going to populate port_c with new
types, and these two functions would become very similar without this
refactoring.

Along the way, let's panic in these methods when we are out of memory -
it's the approach we are taking right now in Tarantool. Corresponding
error injection is dropped as well.

NO_TEST=refactoring
NO_CHANGELOG=refactoring
NO_DOC=refactoring