[buffer] Fix crash

src/hb-buffer-serialize.cc

diff --git a/src/hb-buffer-serialize.cc b/src/hb-buffer-serialize.cc
index 6a19c43..1147194 100644
--- a/src/hb-buffer-serialize.cc
+++ b/src/hb-buffer-serialize.cc
@@ -182,7 +182,7 @@
     } else
       return i - start;
 
-    if (flags & HB_BUFFER_SERIALIZE_FLAG_NO_ADVANCES)
+    if (pos && (flags & HB_BUFFER_SERIALIZE_FLAG_NO_ADVANCES))
     {
       x += pos[i].x_advance;
       y += pos[i].y_advance;
@@ -268,7 +268,7 @@
     } else
       return i - start;
 
-    if (flags & HB_BUFFER_SERIALIZE_FLAG_NO_ADVANCES)
+    if (pos && (flags & HB_BUFFER_SERIALIZE_FLAG_NO_ADVANCES))
     {
       x += pos[i].x_advance;
       y += pos[i].y_advance;