stats(3): Improve t-digest merging of samples which result in mu adjustment underflow.

Allow the calculation of the mu adjustment factor to underflow instead of
rejecting the VOI sample from the digest and logging an error. This trades off
some (currently unquantified) additional centroid error in exchange for better
fidelity of the distribution's density, which is the right trade off at the
moment until follow up work to better handle and track accumulated error can be
undertaken.

Obtained from: Netflix
MFC after: immediately

(cherry picked from commit 1eb402e47af35b3980e6bd51ec462de3a3faa2c8)