Exercise both in-memory BufReader and file-backed FileReader in Decbu…

…f tests.

This revealed two differences in behaviour between BufReader and
FileReader:
* creating a FileReader for an empty file would fail with an error,
  whereas creating a BufReader for an empty buffer would succeed (and
  subsequent reads would fail with an 'invalid size' error)
* reading from a FileReader that had reached the end of the file would
  return a nil byte slice, whereas reading from a BufReader that had
  reached the end of its buffer would return an empty byte slice

Now, both implementations will fail at creation if given an empty
source file / buffer, and both will return an empty slice if reading
from the end of the source file / buffer.

pkg/storegateway/indexheader/encoding/encoding.go

@@ -36,7 +36,13 @@ type Decbuf struct {
 }
 
 func NewDecbufAt(bs ByteSlice, off int, castagnoliTable *crc32.Table) Decbuf {
-	return NewDecbuf(NewBufReader(bs), off, castagnoliTable)
+	reader, err := NewBufReader(bs)
+
+	if err != nil {
+		return Decbuf{E: err}
+	}
+
+	return NewDecbuf(reader, off, castagnoliTable)
 }
 
 // NewDecbufAt returns a new decoding buffer. It expects the first 4 bytes
@@ -137,7 +143,13 @@ func NewDecbufUvarintAt(bs ByteSlice, off int, castagnoliTable *crc32.Table) Dec
 }
 */
 func NewDecbufRaw(bs ByteSlice) Decbuf {
-	return Decbuf{r: NewBufReader(bs)}
+	reader, err := NewBufReader(bs)
+
+	if err != nil {
+		return Decbuf{E: err}
+	}
+
+	return Decbuf{r: reader}
 }
 
 func NewDecbufRawReader(r Reader) Decbuf {