term: erase-scrollback: handle non-existing scrollback history

If scrollback.lines == 0, and the window size (number of rows) is a
power of two, all rows are always visible. I.e. there is no scrollback
history.

This threw off the scrollback erase logic, causing visible rows to be
erased, and set to NULL. This triggered a crash when trying to update
the view.

Closes #1610

CHANGELOG.md

@@ -109,11 +109,16 @@
   -E,--client-environment` ([#1568][1568]).
 * XDG toplevel protocol violation, by trying to set a title that
   contains an invalid UTF-8 sequence ([#1552][1552]).
+* Crash when erasing the scrollback, when scrollback history is
+  exactly 0 rows. This happens when `[scrollback].line = 0`, and the
+  window size (number of rows) is a power of two (i.e. 2, 4, 8, 16
+  etc) ([#1610][1610]).
 
 [1531]: https://codeberg.org/dnkl/foot/issues/1531
 [1573]: https://codeberg.org/dnkl/foot/issues/1573
 [1568]: https://codeberg.org/dnkl/foot/issues/1568
 [1552]: https://codeberg.org/dnkl/foot/issues/1552
+[1610]: https://codeberg.org/dnkl/foot/issues/1610
 
 
 ### Security

terminal.c

@@ -2350,6 +2350,10 @@ term_erase_scrollback(struct terminal *term)
     const int num_rows = grid->num_rows;
     const int mask = num_rows - 1;
 
+    const int scrollback_history_size = num_rows - term->rows;
+    if (scrollback_history_size == 0)
+        return;
+
     const int start = (grid->offset + term->rows) & mask;
     const int end = (grid->offset - 1) & mask;
 
@@ -2416,6 +2420,13 @@ term_erase_scrollback(struct terminal *term)
     }
 
     term->grid->view = term->grid->offset;
+
+#if defined(_DEBUG)
+    for (int i = 0; i < term->rows; i++) {
+        xassert(grid_row_in_view(term->grid, i) != NULL);
+    }
+#endif
+
     term_damage_view(term);
 }