imapc: Update mail size also when istream_opened() hook is called

imapc: Update mail size also when istream_opened() hook is called

When fetching a message body the cached message size was set. However,
if istream_opened() hook was used this wasn't done. If a client had
fetched both (RFC822.SIZE BODY.PEEK[]) and the server sent a wrong
RFC822.SIZE, that size was used for the message body size, which resulted
in "Cached message size larger than expected" errors.