In Coverage iterator, bail out if table smells

In particular, if CoverageFormat2 has unsorted ranges, bail out.
Otherwise, 64k ranges of each 64k glyphs can DoS closure() method.

We can do the same for CoverageFormat1, but that one does not expose
the quadratic behavior, so, fine.

src/hb-ot-layout-common-private.hh

@@ -832,7 +832,12 @@ struct CoverageFormat2
       c = &c_;
       coverage = 0;
       i = 0;
-      j = c->rangeRecord.len ? c_.rangeRecord[0].start : 0;
+      j = c->rangeRecord.len ? c->rangeRecord[0].start : 0;
+      if (unlikely (c->rangeRecord[0].start > c->rangeRecord[0].end))
+      {
+        /* Broken table. Skip. */
+        i = c->rangeRecord.len;
+      }
     }
     inline bool more (void) { return i < c->rangeRecord.len; }
     inline void next (void)
@@ -842,7 +847,14 @@ struct CoverageFormat2
         i++;
 	if (more ())
 	{
+	  hb_codepoint_t old = j;
 	  j = c->rangeRecord[i].start;
+	  if (unlikely (j <= old))
+	  {
+	    /* Broken table. Skip. Important to avoid DoS. */
+	   i = c->rangeRecord.len;
+	   return;
+	  }
 	  coverage = c->rangeRecord[i].value;
 	}
 	return;
@@ -855,7 +867,8 @@ struct CoverageFormat2
 
     private:
     const struct CoverageFormat2 *c;
-    unsigned int i, j, coverage;
+    unsigned int i, coverage;
+    hb_codepoint_t j;
   };
   private: