Since the final size is already known, no need to `ReadAll` a `LimitedReader`: directly `ReadFull` a properly sized buffer.
Tests are already present in `blob_test.go` (a failure can be triggered by creating a smaller `buf`).
`go test -run=TestBlob_Data -bench=Blob_Data -benchmem` before:
```
Benchmark_Blob_Data-18 43964 28727 ns/op 1373 B/op 11 allocs/op
```
After:
```
Benchmark_Blob_Data-18 41308 27679 ns/op 846 B/op 10 allocs/op
```
🎉 one allocation spared!
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12795
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
