Fix bug in bug handling code in packaging script

When no files are found where there should be on the VS packaging script
throw an exception - using a non-existent variable.

Resolving the missing files is a separate problem that I have filed a VS
bug for:
https://developercommunity.visualstudio.com/content/problem/67864/vs-2017-update-3-preview-2-is-missing-mfc-redist-f.html

Bug: 683729
Change-Id: I1a3ba2a342ce7f8fa826300bb808e87c36969b52
Reviewed-on: https://chromium-review.googlesource.com/532114
Reviewed-by: Scott Graham <scottmg@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>

win_toolchain/package_from_installed.py

@@ -60,9 +60,10 @@ def GetVSPath():
 
 def ExpandWildcards(root, sub_dir):
   # normpath is needed to change '/' to '\\' characters.
-  matches = glob.glob(os.path.normpath(os.path.join(root, sub_dir)))
+  path = os.path.normpath(os.path.join(root, sub_dir))
+  matches = glob.glob(path)
   if len(matches) != 1:
-    raise Exception('%s had %d matches - should be one' % (full, len(matches)))
+    raise Exception('%s had %d matches - should be one' % (path, len(matches)))
   return matches[0]