reject out-of-range enum index in read_value

[rootvector2]

What is the purpose of the change

read_value reads an enum's symbol index from the wire with read_long and stores it through avro_value_set_enum without checking it against the schema's symbol count, so a negative or out-of-range index from an untrusted Avro file or datum is kept as-is and later reaches avro_schema_enum_get, where an st_lookup miss leaves the returned char * uninitialized and the json writer dereferences it. The union branch directly above already range-checks its discriminant, and the C++ ValidatingDecoder does the same for enums, so apply the matching check and return EINVAL before the bad index is stored.

Verifying this change

This change added tests and can be verified as follows:

• extended the test_avro_values enum test to read an out-of-range symbol index ({0xC6, 0x01} against a 4-symbol enum); it fails on the unpatched tree and passes with the fix, and the full ctest suite stays green

Documentation

• Does this pull request introduce a new feature? no