releasetools: Fix the bug in AssertSequenceGood.

We use a bitset for blocks in the target image to assert a block hasn't
been touched before reading. Skip checking the blocks that are in the
source image only.

Bug: 27556903
Change-Id: I3a77292da673c813bd20d8dc177ff36419d8ecae
(cherry picked from commit dca2200c8a)

tools/releasetools/blockimgdiff.py

@@ -771,7 +771,10 @@ class BlockImageDiff(object):
           x = x.subtract(sr)
 
       for s, e in x:
-        for i in range(s, e):
+        # Source image could be larger. Don't check the blocks that are in the
+        # source image only. Since they are not in 'touched', and won't ever
+        # be touched.
+        for i in range(s, min(e, self.tgt.total_blocks)):
           assert touched[i] == 0
 
       # Check that the output blocks for this transfer haven't yet