patch by Alvin for 418798

diff --git a/src/live_effects/spiro.cpp b/src/live_effects/spiro.cpp
index 4aad25f08f698f9753d45f1d7ecd53dcc4aa7d0d..abc9c94cac3dde2725dc17eb24914ae79f577e87 100644 (file)
--- a/src/live_effects/spiro.cpp
+++ b/src/live_effects/spiro.cpp
@@ -731,8 +731,10 @@ spiro_iter(spiro_seg *s, bandmat *m, int *perm, double *v, int n)
        add_mat_line(m, v, derivs[1][1], -ends[1][1], 1, j, jk0r, jinc, nmat);
        add_mat_line(m, v, derivs[2][1], -ends[1][2], 1, j, jk1r, jinc, nmat);
        add_mat_line(m, v, derivs[3][1], -ends[1][3], 1, j, jk2r, jinc, nmat);
-       v[jthl] = mod_2pi(v[jthl]);
-       v[jthr] = mod_2pi(v[jthr]);
+       if (jthl >= 0)
+               v[jthl] = mod_2pi(v[jthl]);
+       if (jthr >= 0)
+               v[jthr] = mod_2pi(v[jthr]);
        j += jinc;
     }
     if (cyclic) {