fuzz_pkcs15_reader: Unbreak strict builds

The error was

fuzz_pkcs15_reader.c: In function ‘fuzz_get_chunk’:
fuzz_pkcs15_reader.c:66:19: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
   66 |     *chunk_size = (uint16_t) data->Data;
      |                   ^
cc1: all warnings being treated as errors

src/tests/fuzzing/fuzz_pkcs15_reader.c

@@ -63,7 +63,7 @@ void fuzz_get_chunk(sc_reader_t *reader, const uint8_t **chunk, uint16_t *chunk_
     }
 
     data->Size -= sizeof *chunk_size;
-    *chunk_size = (uint16_t) data->Data;
+    *chunk_size = (uint16_t) *data->Data;
     data->Data += sizeof *chunk_size;
     *chunk = data->Data;