jstestfuzz set a negative number for maxChunkSize in the splitVector command, which the command tries to bit shift left and runs into a runtime error. I think ensuring maxChunkSize is positive would fix the problem.

Might also consider max long long values, wherein bit shifting might exceed the max value? Calling BSONElement::safeNumberLong() ensures we handle out of bound values for maxChunkSize, but then the code bit-shifts it further.