Avoid possible integer overflows computing indexes in OT layout

Author: alerque

src/hb/ot_layout_gsubgpos.rs

@@ -170,7 +170,7 @@ pub fn match_lookahead(
     start_index: usize,
     end_index: &mut usize,
 ) -> bool {
-    let mut iter = skipping_iterator_t::new(ctx, start_index - 1, true);
+    let mut iter = skipping_iterator_t::new(ctx, start_index.saturating_sub(1), true);
     iter.set_glyph_data(0);
     iter.enable_matching(match_func);
 
@@ -867,7 +867,9 @@ fn apply_lookup(
             match_positions[j] = (match_positions[j] as isize + delta) as _;
         }
 
-        backtrack_len + match_end - ctx.buffer.idx
+        backtrack_len
+            .saturating_add(match_end)
+            .saturating_sub(ctx.buffer.idx)
     };
 
     for record in lookups {
@@ -1316,7 +1318,9 @@ pub fn ligate_input(
                 if this_comp == 0 {
                     this_comp = last_num_comps;
                 }
-                let new_lig_comp = comps_so_far - last_num_comps + this_comp.min(last_num_comps);
+                let new_lig_comp = comps_so_far
+                    .saturating_add(this_comp.min(last_num_comps))
+                    .saturating_sub(last_num_comps);
                 _hb_glyph_info_set_lig_props_for_mark(cur, lig_id, new_lig_comp);
             }
             buffer.next_glyph();
@@ -1344,7 +1348,9 @@ pub fn ligate_input(
                 break;
             }
 
-            let new_lig_comp = comps_so_far - last_num_comps + this_comp.min(last_num_comps);
+            let new_lig_comp = comps_so_far
+                .saturating_add(this_comp.min(last_num_comps))
+                .saturating_sub(last_num_comps);
             _hb_glyph_info_set_lig_props_for_mark(info, lig_id, new_lig_comp)
         }
     }