Fixed crash in PyFFContour_GetSplineAfterPoint().

When contour.getSplineAfterPoint(0) is called and contour.is_quadratic
is false, it causes the array self->points[] to be accessed with an
negative index, resulting in a crash or invalid data being used.  The
original code attempted to handle this condition but did so
incorrectly.  Now fixed.
Fixed incorrect conditional test in PyFFContour_GetSplineAfterPoint()

When contour.getSplineAfterPoint() is called and
contour.is_quadratic() is false, the point corresponding to the given
point number is examined and if point.on_curve is true, the point
number is decremented, and if point.on_curve is still true,
decremented again.  The author presumably intended to handle
the situation in which the given point was actually a control point,
in which case both of those tests need to be reversed, testing
point.on_curve for false.  Fixed.
Integers being incorrectly passed to Py_BuildValue() expecting double.

Py_BuildValue() takes a variable-length list of arguments whose types
are determined by a format string.  This means that the compiler
cannot perform the usual type checking and size conversions that it
would normally do.  In this case, an integer zero was being passed as
an argument for which the format specifier was "d", meaning that a C
double was expected.  On machines where int and double are not the
same width, this guarantees corruption of arguments on the stack.

The fix is to substitute 0.0 for 0 in these arguments, which C treats
as constant of type double.
Merge pull request #1044 from blipvert/fixes/17

Fixed crashes and data corruption in PyFFContour_GetSplineAfterPoint()