stats.lua: graphs: fix bad rendering due to division by 0

This fixes two potential divisions by 0 at generate_graph(...):
- If v_avg is (given and) 0.
- if v_max is 0.

The former doesn't seem to happen in practice because v_avg is only
used at one generate_gpah call, where it's apparently not 0.

The latter triggers if all the graph values are 0 (hence v_max is
also 0).

The implication of these divisions by 0 is an invalid y-value which
ends up at the ASS coordinates string for the graph inner content.

On linux the value ends as "nan" (luajit) or "-nan" (lua 5.1/5.2), and
on Windows it's "nan" (luajit) or "-1.#IND00" (lua 5.1/5.2), maybe due
to msvcrt's snprintf.

All these strings are wrong as ASS numbers, but due to luck in how
libass parses the coordinates, "nan" and "-nan" result in empty graph
(which looks OK for "all values are 0"), while "-1.#IND00" is parsed
as -1, which breaks the graph rendering (affects other graphs too).

One example of "all values are 0" is at page 0 (internal performance
graphs) on Windows - because the cpu metrics don't work.
So this fixes at least page 0 on Windows with lua 5.1/5.2.

While at it, move the scale calculations to one place, which now
avoids division by 0, instead of duplicating this calculation.

In the future, we can consider improving the generate_graph API:
- It needs one peak value, but takes 3 (v_max, v_avg, scale) which
  are meshed into one final value.
- v_avg is only used in 1 of 6 call sites, but at the middle of the
  arguments, so all other call sites need to pass explicit "nil".
- "scale" is arbitrary and used to leave some space at the top of the
  graph. 5 places use 0.8, one uses 0.9. Could probably be unified.

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