fixed compile issue when using Halley's Method to refine the inverse of
the cumulative normal distribution.
Prevent some errors when linking a null term structure.

When settings a null term structure to a Handle used as an underlying
for another curve (say, a zero-spreaded curve), the latter tries to
reset the jumps in the base class and fails. This error is now trapped.

Thanks to Christoph Breig for the heads-up.
Merge pull request #50.
add sanity check for left extrapolation
Merge pull request #51.