Fix driver_tools.py to handle the case when command file is empty.

pnacl/driver/driver_tools.py

 #!/usr/bin/python
 # Copyright (c) 2012 The Native Client Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import platform
 import os
 import random
 import re
 import shlex
@@ skipping 342 matching lines @@
 def ParseArgsBase(argv, patternlist):
   """ Parse argv using the patterns in patternlist
       Returns: (matched, unmatched)
   """
   matched = []
   unmatched = []
   i = 0
   while i < len(argv):
     if ShouldExpandCommandFile(argv[i]):
       argv = DoExpandCommandFile(argv, i)
+      if i >= len(argv):
+        break
     num_matched, action, groups = MatchOne(argv, i, patternlist)
     if num_matched == 0:
       unmatched.append(argv[i])
       i += 1
       continue
     matched += argv[i:i+num_matched]
     if isinstance(action, str):
       # Perform $N substitution
       for g in xrange(0, len(groups)):
         action = action.replace('$%d' % g, 'groups[%d]' % g)
@@ skipping 507 matching lines @@
         # Last step
         step_output = self.output
       else:
         # Intermediate step
         if self.use_names_for_input:
           step_output = self.namegen.TempNameForInput(self.input, output_type)
         else:
           step_output = self.namegen.TempNameForOutput(output_type)
       callback(step_input, step_output, **extra)
       step_input = step_output