[buffer] Fix out-buffer under memory-alloc failure

This was broken in July refactoring of the buffer, and exposed to
ReverseChainSingleSubstFormat1 in 3807061d63

Fixes:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=38800
https://bugs.chromium.org/p/chromium/issues/detail?id=1303552

src/hb-buffer.cc

@@ -404,6 +404,7 @@ hb_buffer_t::sync ()
 
 reset:
   have_output = false;
+  out_info = info;
   out_len = 0;
   idx = 0;
 }