AVRO-3194: [c++] Throw instead of segfaulting on GenericDatum::value<T>() type mismatch

[iemejia]

What changes were proposed in this pull request?

GenericDatum::value<T>() (both the mutable and const overloads) returned:

return (type_ == AVRO_UNION) ? std::any_cast<GenericUnion>(&value_)->datum().value<T>()
                             : *std::any_cast<T>(&value_);

The pointer form of std::any_cast<T>(&value_) returns nullptr when the requested C++ type T does not match the type actually held by the datum. Dereferencing that result is an unchecked null-pointer dereference (CWE-476): undefined behaviour that manifests as a segmentation fault as soon as the returned reference is read or copied.

This is the crash reported in AVRO-3194, where user code that binds/copies the result — e.g. const avro::GenericRecord record = datum.value<avro::GenericRecord>() when the datum is not actually a GenericRecord — segfaults instead of receiving a diagnosable error.

How was this patch fixed?

Both overloads now check the any_cast result and throw an avro::Exception describing the datum type on a mismatch, instead of dereferencing a null pointer. Correct-type access and the union-unwrapping path are unchanged.

How was this patch tested?

New regression case testGenericDatumValueTypeMismatch in test/unittest.cc:

• correct-type access still returns the value;
• mismatched-type access (value<int32_t>() / value<std::vector<uint8_t>>() on a string datum) now throws avro::Exception through both the mutable and const overloads.

Verified against the current code: without the fix the test reports exception avro::Exception expected but not raised (the null reference is UB and segfaults when the value is copied, as in the report); with the fix the full unittest suite passes (*** No errors detected).