Fix race condition and Delta Operation incomplete generation

[mdouchement]

Fix #3

• The race condition along with the different sizes of buff initialization was an issue
• As stated in Delta Operation incomplete generation #3, when n, err = io.ReadAtLeast(source, buffer[validTo:validTo+r.BlockSize], r.BlockSize) ended on an io.EOF, n equals 0 and made the remaining data not written.

$ go test -run=NONE -fuzz=FuzzSync -race
fuzz: elapsed: 0s, gathering baseline coverage: 0/76 completed
fuzz: elapsed: 0s, gathering baseline coverage: 76/76 completed, now fuzzing with 16 workers
fuzz: elapsed: 3s, execs: 22875 (7624/sec), new interesting: 7 (total: 83)
fuzz: elapsed: 6s, execs: 51585 (9567/sec), new interesting: 9 (total: 85)
fuzz: elapsed: 9s, execs: 78026 (8802/sec), new interesting: 9 (total: 85)
...
fuzz: elapsed: 10m6s, execs: 6970026 (11394/sec), new interesting: 22 (total: 110)
fuzz: elapsed: 10m9s, execs: 7004121 (11363/sec), new interesting: 22 (total: 110)
fuzz: elapsed: 10m12s, execs: 7021020 (6222/sec), new interesting: 22 (total: 110)
PASS
ok      github.com/minio/rsync-go       613.028s