Closes#1734
Summary:
If you start a skill like fishing lobster after logging in,
you will have the xpGained value as `90`.
But, since you just started the skill, it will divide by `0`.
This results in the max integer value of `2147483647`.
If you did fletching before fishing, and had a XP per hour of `40000`.
Then when you add the fishing XP per hour of `2147483647`, the
result overflows and you get a negative number as the total XP per hour.
Solution:
Don't divide by zero, instead suppose that the skill has been active
for **at least** 60 seconds. This will result in a fair estimate that
improves as the skill continues to be used, rather than the ridiculous
estimate of 2 Billion one second and 200K the next second.
60 seconds is chosen over 1 or 5 or 10 seconds as the result is more
conservative and closer matches my hourly XP experiences.
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