prbuf: fix prbuf_open for empty buffer

prbuf_check, which is called by prbuf_open, proceeds to scanning the
buffer even if it's empty. On debug build, this results in prbuf_open
reporting that the buffer is corrupted, because we trash the buffer in
prbuf_create. On a release build, this may lead to a hang, in case the
buffer is zeroed out. Let's fix this by returning success from
prbuf_check if the buffer is empty. Note, prbuf_iterator_next doesn't
call prbuf_first_record if the buffer is empty, either.

Needed for https://github.com/tarantool/tarantool-ee/issues/187

NO_DOC=bug fix
NO_CHANGELOG=will be added to EE