[sanitizer] Add reset_object(), make set_object() do bounds-check

Affects morx/kerx run-time only currently.  Will adjust their sanitize next.

src/hb-aat-layout-kerx-table.hh

@@ -934,6 +934,7 @@ struct KerxTable
       st = &StructAfter<SubTable> (*st);
       c->set_lookup_index (c->lookup_index + 1);
     }
+    c->sanitizer.reset_object ();
 
     return ret;
   }

src/hb-aat-layout-morx-table.hh

@@ -1041,6 +1041,7 @@ struct Chain
       subtable = &StructAfter<ChainSubtable<Types> > (*subtable);
       c->set_lookup_index (c->lookup_index + 1);
     }
+    c->sanitizer.reset_object ();
   }
 
   inline unsigned int get_size (void) const { return length; }

src/hb-machinery.hh

@@ -259,26 +259,34 @@ struct hb_sanitize_context_t :
 
   inline void set_max_ops (int max_ops_) { max_ops = max_ops_; }
 
-  /* TODO
-   * This set_object() thing is to use sanitize at runtime lookup
-   * application time.  This is very distinct from the regular
-   * sanitizer operation, so, eventually, separate into another
-   * type and make hb_aat_apply_context_t use that one instead
-   * of abusing this one.
-   */
   template <typename T>
   inline void set_object (const T& obj)
   {
-    this->start = (const char *) &obj;
+    reset_object ();
-    this->end = (const char *) &obj + obj.get_size ();
+
+    const char *obj_start = (const char *) &obj;
+    const char *obj_end = (const char *) &obj + obj.get_size ();
+    assert (obj_start <= obj_end); /* Must not overflow. */
+
+    if (unlikely (obj_end < this->start || this->end < obj_start))
+      this->start = this->end = nullptr;
+    else
+    {
+      this->start = MAX (this->start, obj_start);
+      this->end   = MIN (this->end  , obj_end  );
+    }
+  }
+
+  inline void reset_object (void)
+  {
+    this->start = this->blob->data;
+    this->end = this->start + this->blob->length;
     assert (this->start <= this->end); /* Must not overflow. */
   }
 
   inline void start_processing (void)
   {
-    this->start = this->blob->data;
+    reset_object ();
-    this->end = this->start + this->blob->length;
-    assert (this->start <= this->end); /* Must not overflow. */
     this->max_ops = MAX ((unsigned int) (this->end - this->start) * HB_SANITIZE_MAX_OPS_FACTOR,
 			 (unsigned) HB_SANITIZE_MAX_OPS_MIN);
     this->edit_count = 0;