AVRO-4270: [C] Fix wrong schema accessor in avro_generic_map_class

[KyleKim107]

What is the purpose of the change

avro_generic_map_class() in lang/c/src/generic.c incorrectly calls
avro_schema_array_items(schema) instead of avro_schema_map_values(schema).
This is a copy-paste bug from the array class implementation.

The wrong accessor reinterprets a MAP schema pointer as an ARRAY schema type.
Under UndefinedBehaviorSanitizer with -fno-sanitize-recover, this triggers a
fatal diagnostic when processing MAP schemas. Without sanitizers the code
silently returns the correct value today because avro_array_schema_t and
avro_map_schema_t share the same memory layout, but this is fragile and will
break if the struct layouts diverge.

This pull request fixes AVRO-4270.

Verifying this change

This change can be verified as follows:

• Built the C library with ASan + UBSan:
  -fsanitize=address,undefined -fno-sanitize-recover=undefined -g -O1
• Ran a custom value_reader_fuzzer targeting avro_value_read() with a PoC
  input that selects a {"type":"map","values":"string"} schema
• Before fix: UBSan aborts in the map value initialization path
• After fix: no UBSan diagnostic; input is processed normally

Discovered by FuzzingBrain @o2lab.

Documentation

• Does this pull request introduce a new feature? (yes / no)
  no
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)
  not applicable