mpstream: fix NULL dereference in `mpstream_encode_double`

`mpstream_encode_double`, apparently, has a typo: the result of
`mpstream_reserve` is checked after encoding the double into the result
buffer — fix it.

Closes tarantool/security#63

NO_DOC=bug fix
NO_CHANGELOG=see NO_TEST
NO_TEST=unlikely to happen because malloc shouldn't normally fail, and
	we don't test other mpstream methods for OOM either

(cherry picked from commit ccf3130c)