Fix edge case in CIRCLE::Intersect that caused a divide-by-zero crash

Also add some additional test cases.

libs/kimath/src/geometry/circle.cpp

@@ -239,6 +239,9 @@ std::vector<VECTOR2I> CIRCLE::Intersect( const CIRCLE& aCircle ) const
     if( d > ( r1 + r2 ) || ( d < ( std::abs( r1 - r2 ) ) ) )
         return retval; //circles do not intersect
 
+    if( d == 0 )
+        return retval; // circles are co-centered. Don't return intersection points
+
     // Equation (3)
     int64_t x = ( ( d * d ) + ( r1 * r1 ) - ( r2 * r2 ) ) / ( int64_t( 2 ) * d );
     int64_t r1sqMinusXsq = ( r1 * r1 ) - ( x * x );

qa/libs/kimath/geometry/test_circle.cpp

@@ -271,13 +271,21 @@ static const std::vector<CIR_CIR_VECPT_CASE> intersect_circle_cases = {
         },
     },
     {
-        "tangent aligned",
+        "tangent aligned, external",
         { { 10, 10 }, 20 },
         { { 10, 50 }, 20 },
         {
             { 10, 30 },
         },
     },
+    {
+        "tangent aligned, internal",
+        { { 10, 10 }, 40 },
+        { { 10, 30 }, 20 },
+        {
+            { 10, 50 },
+        },
+    },
     {
         "no intersection",
         { { 10, 10 }, 20 },
@@ -318,6 +326,14 @@ static const std::vector<CIR_CIR_VECPT_CASE> intersect_circle_cases = {
             //no points
         },
     },
+    {
+        "Co-centered, same radius", // Exercise d=0
+        { { 205999999, 136367974 }, 3742026 },
+        { { 205999999, 136367974 }, 3742026 },
+        {
+            //no points
+        },
+    },
 };
 // clang-format on