MB-24868: Simplify usage of GetValue in Evp get_if
MB-24868: Simplify usage of GetValue in Evp get_if

This patch helps to avert a potential GCC <= 4.8 bug that causes the
return EngineErrorItemPair to be incorrectly constructed with a nullptr
when compile with O2 and above.

This issue affects all platforms that use GCC 4.8 or below which
includes but is not limited to:
* Ubuntu 14.04
* CentOS 7
* CentOS 6
* Debian 7
Change-Id: I92b6fc32b481edb21598cb37ad1e27bf7797d8e5
Reviewed-on: http://review.couchbase.org/79670
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine