ArrayPoolBufferWriter buffer is used after dispose in Ascii85, AsciiHex and RunLength filters

[BobLd]

The ArrayPoolBufferWriter buffer is used after being dispose in Ascii85, AsciiHex and RunLength filters.

This means the buffer can already be being re-used when the output is used.

2 ways to replicate the issue:

• Set the clearArray parameter to true in ArrayPool<T>.Shared.Return(buffer, true);. The filters corresponding filters tests will fail.
• Run parallel loops of decoding, e.g. in Ascii85FilterTests:

            Parallel.For(0, 100000, (i) =>
            {
                if (i % 2 == 0)
                {
                    var bytes = Encoding.ASCII.GetBytes("9jqo^zBlbD-");

                    var result = filter.Decode(bytes, dictionary, 1);

                    var text = Encoding.ASCII.GetString(result.ToArray());

                    Assert.Equal("Man \0\0\0\0is d", text);
                }
                else
                {
                    const string input =
                        @"0d&.mDdmGg4?O`>9P&*SFD)dS2E2gC4pl@QEb/Zr$8N_r$:7]!01IZ=0eskNAdU47<+?7h+B3Ol2_m!C+?)#1+B1
`9>:<KhASu!rA7]9oF*)G6@;U'.@ps6t@V$[&ART*lARTXoCj@HP2DlU*/0HBI+B1r?0H_r%1a#ac$<nof.3LB""+=MAS+D58'ATD3qCj@.
           F@;@;70ea^uAKYi.Eb-A7E+*6f+EV:*DBN1?0ek+_+B1r?<%9""=ASu!rA7]9oF*)G6@;U'<.3MT)$8<SS1,pCU6jd-H;e7
C#1,U1&Ft""Og2'=;YEa`c,ASu!rA8,po+Dk\3BQ%F&+CT;%+CQ]A1,'h!Ft""Oh2'=;UBl%3eCh4`'DBMbD7O]H>0H_br.:""&q8d[6p/M
T()<(%'A;f?Ma+CT;%+E_a:A0>K&EZek1D/aN,F)u&6DBNA*A0>f4BOu4*+EM76E,9eK+B3(_<%9""p.!0AMEb031ATMF#F<G%,DIIR2+Cno
&@3B9%+CT.1.3LK*+=KNS6V0ilAoD^,@<=+N>p**=$</Jt-rY&$AKYo'+EV:.+Cf>,E,oN2F(oQ1+D#G#De*R""B-;&&FD,T'F!+n3AKY4b
F*22=@:F%a+=SF4C'moi+=Li?EZeh0FD)e-@<>p#@;]TuBl.9kATKCFGA(],AKYo5BOu4*+CT;%+C#7pF_Pr+@VfTuDf0B:+=SF4C'moi+=
Li?EZek1DKKT1F`2DD/TboKAKY](@:s.m/h%oBC'mC/$>""*cF*)G6@;Q?_DIdZpC&~>";

                    var result = filter.Decode(Encoding.ASCII.GetBytes(input), dictionary, 0);

                    var text = Encoding.ASCII.GetString(result.ToArray());

                    Assert.Equal(PdfContent.Replace("\r\n", "\n"), text);
                }
            });

the bytes of one case will be found in the second case.

The solution is to create a copy of the buffer before returning.