fixed: errors when duration or offset are nil

Requests that did not return a value for duration or offset (like pause) would cause an error due to the fact that at least self.duration and/or self.offset were nil. I fixed that by checking setting self.minutes to 0 if either one of those is nil.

I made this request earlier, but due to a snafu with the line endings, it was showing the entire file as being modified. This request shows only the single line being modified.