MINOR: [c++] validate union and enum index in the resolving reader

[arib06]

What is the purpose of the change

ResolverSchema / ResolvingReader is the path used to read a datum written with an older writer schema into an avrogencpp-generated struct, so both the writer schema and the bytes come from the data file. The union branch index and the enum ordinal are plain zigzag varints in that datum, and Reader is ReaderImpl<NullValidator>, so nothing upstream range-checks them before Resolver.cc uses them to index resolvers_, choiceMapping_ and mapping_. Two bytes (90 4e) select branch 5000 of a two-branch union; the out-of-bounds Resolver* that comes back is then virtual-dispatched. The enum site did have a bound, but as an assert, which Release builds drop with -DNDEBUG, and there the out-of-bounds heap word is also copied into the caller's deserialized object. I hit this while feeding malformed datums through the resolving reader under ASan, which reports a heap-buffer-overflow read of 8 bytes at Resolver.cc:332 and Resolver.cc:312. The check now lives next to each index, next to where the value is read, so all four resolver variants reject the branch instead of trusting it.

Verifying this change

This change added tests and can be verified as follows:

• Added TestBadStuff::testOutOfRangeUnionIndex and TestBadStuff::testOutOfRangeEnumIndex in test/unittest.cc, which drive a resolving reader with an out-of-range branch index and ordinal and expect avro::Exception. Without the patch ./build/unittest dies with a memory access violation in the first of them.
• ./build.sh test and a CMAKE_BUILD_TYPE=Release build both pass.

Documentation

• Does this pull request introduce a new feature? no