exponential distribution: Fix fromSample.

[lorinder]

fromSample had a typo causing it to reject all valid samples (those contaning only nonnegative values), and accept only invalid ones.

[Shimuuar]

Thank you! I merged PR separately in #191. One thing correct way to check sample is:

    | G.all (>= 0) xs = Just $! ED (S.mean xs)
    | otherwise       = Nothing

This way we can reject NaNs.

Fix uploaded in 0.16.1.2