Check that number of points passed to wxDC::DrawSpline() is at least 2

DoDrawSpline() implementations for all ports work for the number of points
>= 2 (for 2 points there is drawn a straight line) so we need to add checks
whether this requirement is met.

See #19172.

interface/wx/dc.h

@@ -714,7 +714,7 @@ public:
     /**
         Draws a spline between all given points using the current pen.
 
-        The number of points must be at least 3 for the spline to be drawn.
+        The number of points must be at least 2 for the spline to be drawn.
 
         @beginWxPerlOnly
         Not supported by wxPerl.