Use memset to initialize compat_st structure

The layout of this structure has changed between libcouchbase versions,
and as such, the first element is actually a char*, and not the
structure itself (despite this, ABI is not broken as libcouchbase
defines a new symbol for using the new structure which is mapped at
compile time).

Change-Id: Id30618ddd63992e7bb9183ba8f00a4b34ca44306
Reviewed-on: http://review.couchbase.org/38219
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>