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

pnacl/driver/driver_tools.py

Index: pnacl/driver/driver_tools.py
diff --git a/pnacl/driver/driver_tools.py b/pnacl/driver/driver_tools.py
index 0cb1701d10731dc43435d11a7709e5152a815c72..1bdb943c8230d488c18e6b8d38ba6033f03e4206 100755
--- a/pnacl/driver/driver_tools.py
+++ b/pnacl/driver/driver_tools.py
@@ -360,6 +360,8 @@ def ParseArgsBase(argv, patternlist):
   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])