mcserver: clarify partial packet handling

This commit rearranges some bits of the try_read() function to make the
logic easier to follow. The code semantics itself are not changed.

Change-Id: Ib3c9ba352d909ed5a133e8ce4d8fcfb6500a5d36
Reviewed-on: http://review.couchbase.org/42549
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>