Combining fuzz_data_producer restrict calls into a single function

tests/fuzz/stream_round_trip.c

@@ -128,8 +128,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *src, size_t size)
     /* Give a random portion of src data to the producer, to use for
     parameter generation. The rest will be used for (de)compression */
     FUZZ_dataProducer_t *producer = FUZZ_dataProducer_create(src, size);
-    size_t producerSliceSize = FUZZ_dataProducer_uint32Range(producer, 0, size);
-    size = FUZZ_dataProducer_contract(producer, producerSliceSize);
+    size = FUZZ_dataProducer_reserveDataPrefix(producer);
 
     size_t neededBufSize;
     neededBufSize = ZSTD_compressBound(size) * 5;