Fix incorrect check on control point count

Check for control point count has become inverted during porting. In original C++ [code], this is expressed as `assert (m_controlPointCount >= 4)` on line 77. This is exactly the opposite of what is currently in Java code.

This was submitted to upstream (RoyAwesome/jlibnoise#3), but @lukespragg suggested that I could have better luck getting it merged here.

[code]: http://libnoise.cvs.sourceforge.net/viewvc/libnoise/noise/src/module/curve.cpp?view=markup