mpi3mr: Replace can't happen DataLength == 0 with an assert

Replace the test for DataLength == 0 with an assert. It can't happen,
but an assert doesn't hurt. Emacs removed some trailing white space too.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D42807

(cherry picked from commit 2361a0056fc16b5e11df2aa948aaa633ac01d685)