Fix GCC 4.9 add default null pointer check

Versions of the GNU Compiler Collection (GCC) greater than or equal to GCC 4.9.0 include changes to code optimization that have been reported to cause problems in when default code optimization settings are used by the compiler.

Beginning with GCC 4.9.0, code optimization in GCC now includes (by default) an optimization which is intended to eliminate unnecessary null pointer comparisons in compiled code.  Unfortunately this optimization removes checks which are necessary the demonstrated effect is to cause unpredictable assertion failures during execution of named, resulting in termination of the server process.

[`CMake/HPHPCompiler.cmake:L29`](https://github.com/facebook/hhvm/pull/2963/files#diff-3e1d1439ed0b241d4bcf9f0f5cb35da4R26)