Fix buffer_diff for empty buffers

If buffers are empty, content type should be ignored.

This fixes last of the failing tests: fuzzed.tests.  Green again!

src/hb-buffer.cc

diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index 6b05d3a..171d101 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -1893,7 +1893,7 @@
 		hb_codepoint_t dottedcircle_glyph,
 		unsigned int position_fuzz)
 {
-  if (buffer->content_type != reference->content_type)
+  if (buffer->content_type != reference->content_type && buffer->len && reference->len)
     return HB_BUFFER_DIFF_FLAG_CONTENT_TYPE_MISMATCH;
 
   hb_buffer_diff_flags_t result = HB_BUFFER_DIFF_FLAG_EQUAL;

src/hb-buffer.h

diff --git a/src/hb-buffer.h b/src/hb-buffer.h
index ffbf66e..1d633f7 100644
--- a/src/hb-buffer.h
+++ b/src/hb-buffer.h
@@ -481,12 +481,12 @@
 
   /* Buffers with different content_type cannot be meaningfully compared
    * in any further detail. */
-  HB_BUFFER_DIFF_FLAG_CONTENT_TYPE_MISMATCH	= 0X0001,
+  HB_BUFFER_DIFF_FLAG_CONTENT_TYPE_MISMATCH	= 0x0001,
 
   /* For buffers with differing length, the per-glyph comparison is not
    * attempted, though we do still scan reference for dottedcircle / .notdef
    * glyphs. */
-  HB_BUFFER_DIFF_FLAG_LENGTH_MISMATCH		= 0X0002,
+  HB_BUFFER_DIFF_FLAG_LENGTH_MISMATCH		= 0x0002,
 
   /* We want to know if dottedcircle / .notdef glyphs are present in the
    * reference, as we may not care so much about other differences in this

util/options.hh

diff --git a/util/options.hh b/util/options.hh
index b24ab0c..9feee2d 100644
--- a/util/options.hh
+++ b/util/options.hh
@@ -414,7 +414,7 @@
     }
 
     bool ret = true;
-    hb_buffer_diff_flags_t diff = hb_buffer_diff (buffer, reconstruction, (hb_codepoint_t) -1, 0);
+    hb_buffer_diff_flags_t diff = hb_buffer_diff (reconstruction, buffer, (hb_codepoint_t) -1, 0);
     if (diff)
     {
       if (error)