do not control malformed symbols;

--- a/src/Pure/General/symbol.scala	Sat Jun 18 18:31:55 2011 +0200
+++ b/src/Pure/General/symbol.scala	Sat Jun 18 18:57:38 2011 +0200
@@ -327,6 +327,6 @@
     def is_symbolic(sym: String): Boolean =
       sym.startsWith("\\<") && sym.endsWith(">") && !sym.startsWith("\\<^")
     def is_controllable(sym: String): Boolean =
-      !is_blank(sym) && !sym.startsWith("\\<^")
+      !is_blank(sym) && !sym.startsWith("\\<^") && !is_malformed(sym)
   }
 }