Fix parenthesis in sanitizeCoordinates()

The first if statement checks it either offset is undefined or zero and maybe we have changedTouches data.
However, in the original parenthesis if offsetX = 0 then the whole expression was true even if the changedTouches was undefined, that way an exception was thrown from the inside of the if block. This fixes this error.
Merge pull request #94 from gergely-ujvari/fix-event-parenthesis

Fix parenthesis in sanitizeCoordinates()