fix pull request #492 - select_do_line of a point

Latent bug:  selection_do_line() for a single point produced
spurious line(s).

I haven't got a test case so am taking this one on faith....

While in there, do some nearby formatting fixups in sp_lev.c.

Fixes #492

src/sp_lev.c

@@ -4600,9 +4600,9 @@ selection_do_gradient(
 /* bresenham line algo */
 void
 selection_do_line(
-xchar x1, xchar y1,
-xchar x2, xchar y2,
-struct selectionvar *ov)
+    xchar x1, xchar y1,
+    xchar x2, xchar y2,
+    struct selectionvar *ov)
 {
     int d0, dx, dy, ai, bi, xi, yi;
 
@@ -4613,7 +4613,6 @@ struct selectionvar *ov)
         xi = -1;
         dx = x1 - x2;
     }
-
     if (y1 < y2) {
         yi = 1;
         dy = y2 - y1;
@@ -4624,7 +4623,10 @@ struct selectionvar *ov)
 
     selection_setpoint(x1, y1, ov, 1);
 
-    if (dx > dy) {
+    if (!dx && !dy) {
+        /* single point - already all done */
+        ;
+    } else if (dx > dy) {
         ai = (dy - dx) * 2;
         bi = dy * 2;
         d0 = bi - dx;
@@ -4655,11 +4657,11 @@ struct selectionvar *ov)
 
 void
 selection_do_randline(
-xchar x1, xchar y1,
-xchar x2, xchar y2,
-schar rough,
-schar rec,
-struct selectionvar *ov)
+    xchar x1, xchar y1,
+    xchar x2, xchar y2,
+    schar rough,
+    schar rec,
+    struct selectionvar *ov)
 {
     int mx, my;
     int dx, dy;
@@ -4698,9 +4700,9 @@ struct selectionvar *ov)
 
 static void
 selection_iterate(
-struct selectionvar *ov,
-select_iter_func func,
-genericptr_t arg)
+    struct selectionvar *ov,
+    select_iter_func func,
+    genericptr_t arg)
 {
     int x, y;