Prevent invalid paths in fracture

We are not always sure of the path validity in fracture.  Nominally, the
Clipper Simplify routine should remove invalid parts but in the case
where it doesn't, we need to recover gracefully, even if the output
polygon is not valid.

Fixes https://gitlab.com/kicad/code/kicad/issues/9199

libs/kimath/src/geometry/shape_poly_set.cpp

@@ -55,6 +55,8 @@
 #include <geometry/shape_segment.h>
 #include <geometry/shape_circle.h>
 
+#include <wx/log.h>
+
 
 SHAPE_POLY_SET::SHAPE_POLY_SET() :
     SHAPE( SH_POLY_SET )
@@ -970,14 +972,11 @@ void SHAPE_POLY_SET::fractureSingle( POLYGON& paths )
 
         int x_min = std::numeric_limits<int>::max();
 
-        for( const VECTOR2I& p : points )
-        {
-            if( p.x < x_min )
-                x_min = p.x;
-        }
-
         for( int i = 0; i < pointCount; i++ )
         {
+            if( points[i].x < x_min )
+                x_min = points[i].x;
+
             // Do not use path.CPoint() here; open-coding it using the local variables "points"
             // and "pointCount" gives a non-trivial performance boost to zone fill times.
             FractureEdge* fe = new FractureEdge( first, points[ i ],
@@ -1015,22 +1014,37 @@ void SHAPE_POLY_SET::fractureSingle( POLYGON& paths )
     while( num_unconnected > 0 )
     {
         int x_min = std::numeric_limits<int>::max();
+        auto it = border_edges.begin();
 
         FractureEdge* smallestX = nullptr;
 
         // find the left-most hole edge and merge with the outline
-        for( FractureEdge* border_edge : border_edges )
+        for( ; it != border_edges.end(); ++it )
         {
+            FractureEdge* border_edge = *it;
             int xt = border_edge->m_p1.x;
 
-            if( ( xt < x_min ) && !border_edge->m_connected )
+            if( ( xt <= x_min ) && !border_edge->m_connected )
             {
                 x_min = xt;
                 smallestX = border_edge;
             }
         }
 
-        num_unconnected -= processEdge( edges, smallestX );
+        int num_processed = processEdge( edges, smallestX );
+
+        // If we can't handle the edge, the zone is broken (maybe)
+        if( !num_processed )
+        {
+            wxLogWarning( "Broken polygon, dropping path" );
+
+            for( FractureEdge* edge : edges )
+                delete edge;
+
+            return;
+        }
+
+        num_unconnected -= num_processed;
     }
 
     paths.clear();