[set] don't allow -1 (HB_SET_VALUE_INVALID) to be inserted into a hb_set_t.

Add tests that check all of the addition methods.

src/hb-bit-set.hh

@@ -194,7 +194,7 @@ struct hb_bit_set_t
       unsigned int end = major_start (m + 1);
       do
       {
-        if (v || page) /* The v check is to optimize out the page check if v is true. */
+        if (g != INVALID && (v || page)) /* The v check is to optimize out the page check if v is true. */
 	  page->set (g, v);
 
 	array = &StructAtOffsetUnaligned<T> (array, stride);
@@ -238,7 +238,7 @@ struct hb_bit_set_t
 	if (g < last_g) return false;
 	last_g = g;
 
-        if (v || page) /* The v check is to optimize out the page check if v is true. */
+        if (g != INVALID && (v || page)) /* The v check is to optimize out the page check if v is true. */
 	  page->add (g);
 
 	array = &StructAtOffsetUnaligned<T> (array, stride);

src/test-set.cc

@@ -137,5 +137,29 @@ main (int argc, char **argv)
     assert (s.has (HB_SET_VALUE_INVALID));
   }
 
+  /* Adding HB_SET_VALUE_INVALID */
+  {
+    hb_set_t s;
+
+    s.add(HB_SET_VALUE_INVALID);
+    assert(!s.has(HB_SET_VALUE_INVALID));
+
+    s.clear();
+    assert(!s.add_range(HB_SET_VALUE_INVALID - 2, HB_SET_VALUE_INVALID));
+    assert(!s.has(HB_SET_VALUE_INVALID));
+
+    hb_codepoint_t array[] = {(unsigned) HB_SET_VALUE_INVALID, 0, 2};
+    s.clear();
+    s.add_array(array, 3);
+    assert(!s.has(HB_SET_VALUE_INVALID));
+    assert(s.has(2));
+
+    hb_codepoint_t sorted_array[] = {0, 2, (unsigned) HB_SET_VALUE_INVALID};
+    s.clear();
+    s.add_sorted_array(sorted_array, 3);
+    assert(!s.has(HB_SET_VALUE_INVALID));
+    assert(s.has(2));
+  }
+
   return 0;
 }