Set bytes(%) to 0.0 if total bytes allocated is 0

Previously, bytes(%) showed up as NaN if total bytes allocated was 0.  This confused me because I thought it was unlikely that my code was allocating nothing so something weird must have been happening. I think 0.0 is a better default, because as [Calvin](http://www.gocomics.com/calvinandhobbes/1995/04/27) says, "if your numbers go ~~up~~ down, it means you're having more fun."