Specify offset 0 as invalid

doc/decompressor_accepted_invalid_data.md

@@ -0,0 +1,14 @@
+Decompressor Accepted Invalid Data
+==================================
+
+This document describes the behavior of the reference decompressor in cases
+where it accepts an invalid frame instead of reporting an error.
+
+Zero offsets converted to 1
+---------------------------
+If a sequence is decoded with `literals_length = 0` and `offset_value = 3`
+while `Repeated_Offset_1 = 1`, the computed offset will be `0`, which is
+invalid.
+
+The reference decompressor will process this case as if the computed
+offset was `1`, including inserting `1` into the repeated offset list.