Allow toXML() to be called on individual tables

Was hitting a missing attribute since disassembleInstructions was
being set by saveXML() only.
diff --git a/Lib/fontTools/ttLib/tables/ttProgram.py b/Lib/fontTools/ttLib/tables/ttProgram.py
index 32e1dd2..fa6ea69 100644
--- a/Lib/fontTools/ttLib/tables/ttProgram.py
+++ b/Lib/fontTools/ttLib/tables/ttProgram.py
@@ -224,7 +224,7 @@
 		return self.assembly
 	
 	def toXML(self, writer, ttFont):
-		if ttFont.disassembleInstructions:
+		if hasattr (ttFont, "disassembleInstructions") and ttFont.disassembleInstructions:
 			assembly = self.getAssembly()
 			writer.begintag("assembly")
 			writer.newline()