Do not die if kernel is too old for full /proc/stat info

[manpen]

LinuxProcStat fails (and dies) on Linux Kernels < 2.6.33 since /proc/stat contains less values than expected.
This small change circumvents the problem by reporting "na" if the value is not availble.

[lorenzhs]

Wow 2.6.32 borders on OS archaeology – do I want to know how you got a C++14 compiler running on that system? ;)
Timo will probably take a look at it.

[bingmann]

There are two problems with this:
a) you have some typos in the LOG messages.
b) I don't think "na" is valid Json. actually, NaN and Inf or Infinity is also not valid json. I would consider outputting 0 is the best choice.

[manpen]

a) Clearly, sorry about that
b) If we already have a NaN why not use it to clearly indicate that something is missing?

[bingmann]

NaN and Inf or Infinity is also not valid json. See wikipedia, set to zero.