change max for f to maxiumum real value (this is just to ensure that the value for f is valid)
It seems enough to ensure that f is a valid real, so set the maximum value to QL_MAX_REAL.
If a numerical problem during interpolation occurs (zero near boundary not found or f is infinite or both)
delete the right point of the interpolation interval and try the next one. If no point is left, continue
with the extrapolation.
Merge branch 'fix20131021'