Reduce crc32 benchmark memory usage

Reset the length of the slice where save the result to recycle the same allocated memory and prevent further allocations.
The Sum function uses append, so the length of the "in" slice increased with each cycle of the benchmark.

src/hash/crc32/crc32_test.go

@@ -329,11 +329,14 @@ func benchmark(b *testing.B, h hash.Hash32, n, alignment int64) {
 	h.Reset()
 	h.Write(data)
 	h.Sum(in)
+	// Avoid further allocations
+	in = in[:0]
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
 		h.Reset()
 		h.Write(data)
 		h.Sum(in)
+		in = in[:0]
 	}
 }