examples: merge: remove useless while condition

Suggested by: shaitan@
examples: merge: check results of read/write

Reported by: shaitan@
examples: merge: introduce -m parameter for max record size

This is usefull for badly broken blobs.
examples: merge: added even more sanity checks

This allows eblob_merge to pass tests with index corrupted by:
    dd if=/dev/urandom
examples: merge: check both header for equality