storaged: Avoid divide-by-zero in performance measurement

The code previously assumed that write() took a non-zero number of clock
ticks.  This is mostly true, but can't be guaranteed.  Add a check to
avoid crashing if it's 0.

Bug: 233008287
Test: Compile
Change-Id: Idec0052f534dc4abb81a414ca76f6c11f4a4dded