MINOR: [ruby] reject out-of-range enum and union index in binary decoder

[arib06]

What is the purpose of the change

read_enum, read_union and skip_union in lang/ruby/lib/avro/io.rb take the enum symbol index and the union branch index straight from untrusted serialized data and index writers_schema.symbols / writers_schema.schemas with no range check. Because Ruby arrays treat a negative subscript as an offset from the end, a crafted negative index (the zigzag byte 0x01 decodes to -1) wraps to a valid but wrong symbol or branch and is decoded silently, which is a type confusion / stream desync rather than a decode failure. A positive out-of-range index yields nil and later raises NoMethodError. The Python decoder already rejects these (AVRO-4296); this brings the Ruby SDK in line by validating 0 <= index < size at the branch point and raising Avro::AvroError.

Verifying this change

This change added tests and can be verified as follows:

• Added test_enum_index_out_of_range and test_union_index_out_of_range to test/test_io.rb, which feed the decoder crafted bytes with a negative index (-1, wraps to the last element) and a positive out-of-range index, asserting Avro::AvroError is raised. Both tests fail on the unpatched decoder (the enum returns the wrong symbol, the union decodes the wrong branch) and pass with the fix.

Documentation

• Does this pull request introduce a new feature? no