[otlayout] Fix up recent Context matching change

Commit 6b65a76b.  "end" was becoming
negative.  Was trigerred by Lohit-Kannada 2.5.3 and the sequence:
U+0CB0,U+200D,U+0CBE,U+0CB7,U+0CCD,U+0C9F,U+0CCD,U+0CB0,U+0C97,U+0CB3
Two glyphs were being duplicated.

src/hb-buffer.cc

@@ -365,8 +365,12 @@ hb_buffer_t::move_to (unsigned int i)
   {
     assert (i <= len);
     idx = i;
+    return true;
   }
-  else if (out_len < i)
+
+  assert (i <= out_len + (len - idx));
+
+  if (out_len < i)
   {
     unsigned int count = i - out_len;
     if (unlikely (!make_room_for (count, count))) return false;
@@ -388,6 +392,7 @@ hb_buffer_t::move_to (unsigned int i)
     out_len -= count;
     memmove (info + idx, out_info + out_len, count * sizeof (out_info[0]));
   }
+
   return true;
 }
 

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

@@ -1031,7 +1031,8 @@ static inline bool apply_lookup (hb_apply_context_t *c,
 
     /* Recursed lookup changed buffer len.  Adjust. */
 
-    end += delta;
+    /* end can't go back past the current match position. */
+    end = MAX ((int) match_positions[idx] + 1, int (end) + delta);
 
     unsigned int next = idx + 1; /* next now is the position after the recursed lookup. */