Fix degenerate-pen test case by removing the triggering assertion

Instead we choose either the first or last pen vertex as
appropriate.

This makes the degenerate-pen pass stop failing on an
assertion, and passes for most backends. It's still failing
for the PDF backend, but that looks like a new, PDF-specific
bug.

src/cairo-pen.c

@@ -323,7 +323,13 @@ _cairo_pen_find_active_cw_vertex_index (cairo_pen_t *pen,
 	    break;
     }
 
-    assert (i < pen->num_vertices);
+    /* If the desired slope cannot be found between any of the pen
+     * vertices, then we must have a degenerate pen, (such as a pen
+     * that's been transformed to a line). In that case, we consider
+     * the first pen vertex as the appropriate clockwise vertex.
+     */
+    if (i == pen->num_vertices)
+	i = 0;
 
     *active = i;
 }
@@ -351,6 +357,14 @@ _cairo_pen_find_active_ccw_vertex_index (cairo_pen_t *pen,
 	    break;
     }
 
+    /* If the desired slope cannot be found between any of the pen
+     * vertices, then we must have a degenerate pen, (such as a pen
+     * that's been transformed to a line). In that case, we consider
+     * the last pen vertex as the appropriate counterclockwise vertex.
+     */
+    if (i < 0)
+	i = pen->num_vertices - 1;
+
     *active = i;
 }