[cff] Fix a typo in processing rlinecurve

We want to process all pairs of arguments except the last 6 as lines, so
should have been subtracting 6 here, otherwise if the number of
arguments happens to be multiples of 6 they will be all treated as
curves.

See https://github.com/harfbuzz/harfbuzz/pull/2016#issuecomment-554640098

src/hb-cff-interp-cs-common.hh

@@ -573,7 +573,7 @@ struct path_procs_t
   static void rlinecurve (ENV &env, PARAM& param)
   {
     unsigned int i = 0;
-    unsigned int line_limit = (env.argStack.get_count () % 6);
+    unsigned int line_limit = (env.argStack.get_count () - 6);
     for (; i + 2 <= line_limit; i += 2)
     {
       point_t pt1 = env.get_pt ();