Check that we have a non-null contour

Inst_eddies/mean_speed.f90

@@ -16,6 +16,7 @@ contains
     ! Libraries:
     use contour_531, only: polyline
     use numer_rec_95, only: bilinear_interp2_reg
+    use nr_util, only: assert
 
     real, intent(in):: u(:, :), v(:, :) ! velocity
     type(polyline), intent(in):: p ! should be closed
@@ -41,6 +42,8 @@ contains
 
     !-------------------------------------------------------------------
 
+    call assert(p%n_points >= 2, &
+         "mean_speed: requires non-null, closed contour")
     ni = p%n_points - 1
     call bilinear_interp2_reg(corner, step, u, v, p%points(:, :ni), ui, vi)