memlat: handle no histograms better
memlat: handle no histograms better

Currently, the Javascript code will throw with a divide-by-zero if
there are no histograms. Make it return cleanly.

aclements/go-perf,aclements/go-perf,aclements/goperf,aclements/goperf,aclements/goperf,aclements/go-perf,aclements/goperf,aclements/go-perf