SERVER-57897's new test case benchrun_read_pref_mode.js just exposed the existing benchRun() memory leak under exception. The memory leak has been fixed at master. The following code should be ported to v5.0/v4.4/v4.2/v4.0.

https://github.com/mongodb/mongo/blob/master/src/mongo/shell/bench.cpp#L386-L390

Lowering the priority to minor since it's an existing memory leak, not a regression.

And this should not be a blocker for any releases.