verify hash of input file in rred

verify hash of input file in rred

We read the entire input file we want to patch anyhow, so we can also
calculate the hash for that file and compare it with what he had
expected it to be.

Note that this isn't really a security improvement as a) the file we
patch is trusted & b) if the input is incorrect, the result will hardly be
matching, so this is just for failing slightly earlier with a more
relevant error message (althrough, in terms of rred its ignored and
complete download attempt instead).

(cherry picked from commit 6e71ec6fcdcaa926c98fa58cd4af38e42556df15)