Don't run step if miniprofiler doesn't exist

[oleg-polivannyi]

For some reason, miniprofiler object isn't accessible in middleware in some conditions

[coveralls]

Coverage decreased (-0.3%) to 99.494% when pulling bf9403c on daniloisr:dont-run-step-if-miniprofiler-does-not-exist into fa9c21b on MiniProfiler:master.

[daniloisr]

We tracked this bug and found that when using body-parser it mutates the req object and the miniprofiler getter becomes undefined (although we didn't find exactly where this happens).

This PR is a patch that don't affect most of timings, as conflict with body-parser only happens on request that uses req.body.

The proper solution for this problem is to refactor minprofiler internals to not depend on req object anymore, because using async_hooks we can access the current Nodejs context without using req, and I'm already working on this.

So this patch PR LGTM 👍