logging: fix unused variable warning in non-debug builds
logging: fix unused variable warning in non-debug builds

Summary:
Use FOLLY_SAFE_DCHECK() rather than assert() to avoid compiler errors about
this variable being unused in builds where assert() statements are disabled.

Reviewed By: bolinfest, yfeldblum

Differential Revision: D6421978

fbshipit-source-id: ab1019fa1fa420856e1bf3bc39c441662c4efd75

rklabs/folly,facebook/folly,facebook/folly,rklabs/folly,rklabs/folly,facebook/folly,rklabs/folly,rklabs/folly,facebook/folly,facebook/folly