decoder: Fix thinko in PROTO opcode handling

[navytux]

Due to := in

v, err := d.r.ReadByte()

newly declared err was shadowing outside err and thus even though the code intent was to treat all protocols != 2 as ErrInvalidPickleVersion, after leaving the switch err was always =nil and no error was reported.

Fix it by explicitly declaring v and not using := not to shadow err.

We also change the condition for supported protocol version to be in [0, 4] because:

• we already have messages in our testsuite with PROTO opcode and version 1, and if we don't adjust the version condition the tests will fail.

• the highest protocol version we support opcodes for is 4.

• even though the documentation for PROTO opcode says version must be >= 2, in practice CPython decodes pickles with PROTO and lower version just ok:

  In [18]: pickle.loads("\x80\x02I5\n.") Out[18]: 5

  In [19]: pickle.loads("\x80\x01I5\n.") Out[19]: 5

  In [20]: pickle.loads("\x80\x00I5\n.") Out[20]: 5

  so we don't complain about those lower versions too.

[kisielk]

Ah, shadowing strikes again :/ Good catch, thanks!

[navytux]

Thanks for merging.