dd: should error on count=B but doesn't

Coreutils dd:
```
$ : | dd count=B
dd: invalid number: ‘B’
```

uutils dd:
```
$ : | ./target/debug/dd count=B
0+0 records in
0+0 records out
0 bytes copied, 2.9087e-04 s, 0.0 B/s
```


This is from the GNU test suite file `tests/dd/bytes.sh`.