2-Pt Persp. select: Avoid div-by-zero crash

h/t Pere

docs/CHANGES.txt

@@ -6,7 +6,7 @@ Copyright (c) 2002-2024
 Various contributors (see below, and AUTHORS.txt)
 https://tuxpaint.org/
 
-2024.January.16 (0.9.32)
+2024.January.20 (0.9.32)
   * Improvements to Magic tools:
     ----------------------------
     * Magic tool plugin API updates
@@ -66,6 +66,7 @@ https://tuxpaint.org/
         - Two "Oblqiue Draw" tools are provided, offering recedings axes
           to both the right and the left.
       + Code: Bill Kendrick <bill@newbreedsoftware.com>
+        (with help from Pere Pujal i Carabantes <perepujal@gmail.com>)
       + Sounds:
         - Select: "Gummibandloop_121.wav"
           (https://freesound.org/people/von_Gardener/sounds/473829/)

magic/src/n_pt_persp.c

@@ -15,8 +15,9 @@
    axon. & oblique drawing modes.
 
    by Bill Kendrick <bill@newbreedsoftware.com>
+   with help from Pere Pujal Carabantes
 
-   December 12, 2023 - January 16, 2024
+   December 12, 2023 - January 20, 2024
 */
 
 #include <stdio.h>
@@ -1527,6 +1528,7 @@ void n_pt_persp_draw_points(magic_api * api, int tool, SDL_Surface * canvas) {
         x2 = 0;
       }
 
+      if (a1_pt_x != x1) {
         slope = ((float) a1_pt_y - (float) y1) / ((float) a1_pt_x - (float) x1);
         y2 = y1 + (x2 - x1) * slope;
 
@@ -1550,9 +1552,14 @@ void n_pt_persp_draw_points(magic_api * api, int tool, SDL_Surface * canvas) {
           }
         }
       }
+    }
   } else if (tool == TOOL_2PT_SELECT) {
     /* 2-point perspective */
 
+    if (abs(a2_pt_x[0] - a2_pt_x[1]) < SNAP) {
+      a2_pt_x[1] = a2_pt_x[0] + SNAP;
+    }
+
     for (i = 0; i < 2; i++) {
       n_pt_persp_draw_one_point(api, canvas, a2_pt_x[i], a2_pt_y[i], i);
     }
@@ -1571,7 +1578,7 @@ void n_pt_persp_draw_points(magic_api * api, int tool, SDL_Surface * canvas) {
     x = (a2_pt_x[0] + a2_pt_x[1]) / 2;
 
     /* Perpendicular-to-horizon line */
-    if (slope == 0.0 || slope == M_PI) {
+    if (slope == 0.0 || slope == M_PI) { // FIXME: M_PI, really? -bjk 2024.01.20
       x1 = x;
       y1 = 0;
       x2 = x;