[Arabic] Fix post-context handling

Ouch!

src/hb-ot-shape-complex-arabic.cc

@@ -286,7 +286,7 @@ arabic_joining (hb_buffer_t *buffer)
   if (!(buffer->flags & HB_BUFFER_FLAG_EOT))
     for (unsigned int i = 0; i < buffer->context_len[1]; i++)
     {
-      unsigned int this_type = get_joining_type (buffer->context[1][i], buffer->unicode->general_category (buffer->context[0][i]));
+      unsigned int this_type = get_joining_type (buffer->context[1][i], buffer->unicode->general_category (buffer->context[1][i]));
 
       if (unlikely (this_type == JOINING_TYPE_T))
 	continue;