Fix buffer underrun in i386-dis.c.

opcodes/
	* i386-dis.c (print_insn): Do not touch all_prefixes[-1] when
	last_rex_prefix is -1.

opcodes/ChangeLog

@@ -1,3 +1,9 @@
+2014-01-09  Bradley Nelson  <bradnelson@google.com>
+	    Roland McGrath  <mcgrathr@google.com>
+
+	* i386-dis.c (print_insn): Do not touch all_prefixes[-1] when
+	last_rex_prefix is -1.
+
 2014-01-08  H.J. Lu  <hongjiu.lu@intel.com>
 
 	* i386-gen.c (process_copyright): Update copyright year to 2014.

opcodes/i386-dis.c

@@ -12645,7 +12645,7 @@ print_insn (bfd_vma pc, disassemble_info *info)
     }
 
   /* Check if the REX prefix is used.  */
-  if (rex_ignored == 0 && (rex ^ rex_used) == 0)
+  if (rex_ignored == 0 && (rex ^ rex_used) == 0 && last_rex_prefix >= 0)
     all_prefixes[last_rex_prefix] = 0;
 
   /* Check if the SEG prefix is used.  */