Sync with upstream 643521d:

853a3e4: Remove redundant check and wrong fix for bad root cluster chain.
41a4c01: Use calloc()
4438d71: Comment boot block checks and perform additional sanity checks.
32e0aae: Remove unneeded blank line (not applicable to Android).
9502e5b: Correct overflow logic in fullpath().
643521d: Avoid mixing cluster numbers and sector numbers.
         Makes code more readable.

Bug: 141949889
Fixed: 141949889
Change-Id: I38d8cf0af2fd4b7168c68ebc5ff337f42e653be8