Fix issue with NULL as_a in duplicate_insn_chain

2014-08-20  David Malcolm  <dmalcolm@redhat.com>

	* cfgrtl.c (duplicate_insn_chain): Convert the checked cast on
	"insn" from an as_a to a safe_as_a, for the case when "insn" is
	NULL.

From-SVN: r214207

gcc/ChangeLog

+2014-08-20  David Malcolm  <dmalcolm@redhat.com>
+
+	* cfgrtl.c (duplicate_insn_chain): Convert the checked cast on
+	"insn" from an as_a to a safe_as_a, for the case when "insn" is
+	NULL.
+
 2014-08-20  Manuel López-Ibáñez  <manu@gcc.gnu.org>
 
 	PR preprocessor/51303

gcc/cfgrtl.c

@@ -4166,7 +4166,7 @@ duplicate_insn_chain (rtx from, rtx to)
     }
   insn = NEXT_INSN (last);
   delete_insn (last);
-  return as_a <rtx_insn *> (insn);
+  return safe_as_a <rtx_insn *> (insn);
 }
 
 /* Create a duplicate of the basic block BB.  */