Fixed negative endLocation bug

[a-v-l]

On sites using tht optional fixed header ('scrollHeader') an anchor might be placed above the fixed header. This leads to an negative 'endLocation' and therefore the interval 'runAnimation' will not be cleared.

[cferdinandi]

@a-v-l - I appreciate the pull request, and wanted to let you know why I didn't implement it. Setting the endLocation variable to 0 when it's negative, you decrease the value of the distance variable, which is used to calculate the easing pattern. The scroll still stops at the right place, but the pattern would be thrown off.

Admittedly, the math behind the easing patterns is not my own, nor my area of expertise. But from looking at what the code is doing, it seems that changing the distance variable throws off the pattern.

Again, appreciate you finding this bug, though. Thanks!

[cferdinandi]

I just realized the error in my logic. The decreased value of the distance variable using your approach is actually more accurate, as it reflects the real distance the animation needs to travel. I'll update accordingly.

Thanks again!