Crash in kstat_print_individual when /proc/cpuinfo contains "cpu MHz : 0.000"

_From [byron.c....@gmail.com](https://code.google.com/u/101184679827294438523/) on June 18, 2014 21:31:18_

What steps will reproduce the problem? 1. Start a linux VirtualBox instance in which /proc/cpuinfo contains "0.000" for the "cpu MHz" field.
2. Run any multi-threaded program under DR
3. DR crashes with "divide by zero" error on thread exit in core/stats.c:kstat_print_individual() because kstat_frequency_per_msec was initialized to zero in kstat_init().

The crash is really caused by a configuration error in the user's OS, but since it is not uncommon (e.g., https://forums.virtualbox.org/viewtopic.php?f=3&t=44491 ), it might be nice to supply a default non-zero value when this field in /proc/cpuinfo is zero.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1463_