Change pack_firmware_image::EntryBlob behavior

utility/pack_firmware_image

Index: utility/pack_firmware_image
diff --git a/utility/pack_firmware_image b/utility/pack_firmware_image
index d6251cd4db1815da8bf281470cd0929779957ea5..7ec8607328c09154a700eb70bfa24f266007eadb 100755
--- a/utility/pack_firmware_image
+++ b/utility/pack_firmware_image
@@ -129,9 +129,9 @@ class EntryBlob(EntryFmapArea):
 
   def Pack(self, firmware_image, entries):
     size = os.stat(self.path).st_size
-    if size > 0:
-      size = min(size, self.length)
-    else:
+    if size > self.length:
+        raise PackError('blob too large: %d > %d' % (size, self.length))
+    if size == 0: # special case for files like /dev/zero
       size = self.length
     with open(self.path, 'rb') as blob_image:
       firmware_image.seek(self.offset)