panic if hash returns an error

Add a sanity check that the interface contract is honoured.

internal/restic/rewind_reader.go

@@ -56,7 +56,10 @@ func NewByteReader(buf []byte, hasher hash.Hash) *ByteReader {
 	var hash []byte
 	if hasher != nil {
 		// must never fail according to interface
-		_, _ = hasher.Write(buf)
+		_, err := hasher.Write(buf)
+		if err != nil {
+			panic(err)
+		}
 		hash = hasher.Sum(nil)
 	}
 	return &ByteReader{

internal/restic/rewind_reader_test.go

@@ -54,7 +54,10 @@ func TestFileReader(t *testing.T) {
 			var hash []byte
 			if hasher != nil {
 				// must never fail according to interface
-				_, _ = hasher.Write(buf)
+				_, err := hasher.Write(buf)
+				if err != nil {
+					panic(err)
+				}
 				hash = hasher.Sum(nil)
 			}
 			rd, err := NewFileReader(f, hash)