terminal: handle empty tabstop ranges

Tabstops.reset subtracted one from the column count before iterating
default stops. Although init and resize accept zero columns, resetting
that state with a nonzero interval underflowed and panicked.

Return after clearing when the grid has fewer than two columns. Empty
and single-column tabstop sets now preserve the normal no-stop result.

src/terminal/Tabstops.zig

@@ -170,11 +170,11 @@ pub fn reset(self: *Tabstops, interval: usize) void {
     @memset(&self.prealloc_stops, 0);
     @memset(self.dynamic_stops, 0);
 
-    if (interval > 0) {
-        var i: usize = interval;
-        while (i < self.cols - 1) : (i += interval) {
-            self.set(i);
-        }
+    if (interval == 0 or self.cols <= 1) return;
+
+    var i: usize = interval;
+    while (i < self.cols - 1) : (i += interval) {
+        self.set(i);
     }
 }
 
@@ -230,6 +230,13 @@ test "Tabstops: interval" {
     try testing.expect(t.get(8));
 }
 
+test "Tabstops: interval with zero columns" {
+    var t: Tabstops = try init(testing.allocator, 0, 8);
+    defer t.deinit(testing.allocator);
+
+    try testing.expectEqual(@as(usize, 0), t.cols);
+}
+
 test "Tabstops: count on 80" {
     // https://superuser.com/questions/710019/why-there-are-11-tabstops-on-a-80-column-console