Fix mdrun hanging upon exit with sep PME ranks

Fix mdrun hanging upon exit with sep PME ranks

Commit 1d2d95e introduced a check and early return to skip printing perf
stats when no valid wallcycle data was collected (due to missed reset).
However, as the validity of wallcycle data does not get checked/recorded
on separate PME ranks, mdrun deadlocks before exit in collective comm
that PME ranks never enter.

This change fixes the hang by refactoring the printing code to use a
boolean rather than an early return. This means the normal code path
is unaffected in all cases (only the simulation master can ever write
reports), and the case where it is invalid to write a report
(premature termination) works correctly because all ranks communicate
the data for the report that is never written (and efficiency is not
of concern in this case).

Fixes #2131

Change-Id: If8b0813444d0b00a1a9a4a21d30fc8655c52752a