support for CEF fonts: don't depend on the head table being available


git-svn-id: svn://svn.code.sf.net/p/fonttools/code/trunk@405 4cde692c-a291-49d1-8350-778aa11640f8
diff --git a/Lib/fontTools/ttLib/sfnt.py b/Lib/fontTools/ttLib/sfnt.py
index bd5fe7a..0ce0ca0 100644
--- a/Lib/fontTools/ttLib/sfnt.py
+++ b/Lib/fontTools/ttLib/sfnt.py
@@ -129,9 +129,13 @@
 		directory = sstruct.pack(sfntDirectoryFormat, self)
 		
 		self.file.seek(sfntDirectorySize)
+		seenHead = 0
 		for tag, entry in tables:
+			if tag == "head":
+				seenHead = 1
 			directory = directory + entry.toString()
-		self.calcMasterChecksum(directory)
+		if seenHead:
+			self.calcMasterChecksum(directory)
 		self.file.seek(0)
 		self.file.write(directory)
 		if closeStream: