Fix the error that free_string is not defined.

Change-Id: I7490a91ea930daaf194dad0c8b3a3357558b4770
Bug:19573713

tools/releasetools/blockimgdiff.py

@@ -294,6 +294,8 @@ class BlockImageDiff(object):
       if stashed_blocks > max_stashed_blocks:
         max_stashed_blocks = stashed_blocks
 
+      free_string = []
+
       if self.version == 1:
         src_string = xf.src_ranges.to_string_raw()
       elif self.version >= 2:
@@ -306,7 +308,6 @@ class BlockImageDiff(object):
 
         size = xf.src_ranges.size()
         src_string = [str(size)]
-        free_string = []
 
         unstashed_src_ranges = xf.src_ranges
         mapped_stashes = []