Fix panic with empty input in LineReaderAt

gitdiff/io.go

···
 			return 0, err
 		}
 	}
-	if startLine > int64(len(r.index)) {
+	if startLine >= int64(len(r.index)) {
 		return 0, io.EOF
 	}
 

gitdiff/io_test.go

···
 			EOF:        true,
 			EOFCount:   4,
 		},
+		"emptyInput": {
+			InputLines: 0,
+			Offset:     0,
+			Count:      2,
+			EOF:        true,
+			EOFCount:   0,
+		},
 		"offsetAfterEOF": {
 			InputLines: 8,
 			Offset:     10,