Fix some confusions in no-dirty-detection mode

This caused some awful crashes, but doesn't affect any real release builds of waterboxhost

src/BizHawk.Common/SpanStream.cs

@@ -34,6 +34,9 @@ namespace BizHawk.Common
 			private readonly Stream _stream;
 			public unsafe int Read(Span<byte> buffer)
 			{
+				if (buffer.Length == 0)
+					return 0;
+
 				if (buffer.Length > _buffer.Length)
 				{
 					_buffer = new byte[buffer.Length];
@@ -48,6 +51,9 @@ namespace BizHawk.Common
 
 			public unsafe void Write(ReadOnlySpan<byte> buffer)
 			{
+				if (buffer.Length == 0)
+					return;
+
 				if (buffer.Length > _buffer.Length)
 				{
 					_buffer = new byte[buffer.Length];

waterbox/waterboxhost/src/memory_block/mod.rs

@@ -817,13 +817,14 @@ impl MemoryBlock {
 
 		self.refresh_all_protections();
 		self.sealed = true;
-		#[cfg(not(feature = "no-dirty-detection"))]
-		{
-			use sha2::{Sha256, Digest};
 
+		use sha2::{Sha256, Digest};
 		self.hash = {
 			let mut hasher = Sha256::new();
 			bin::write(&mut hasher, &self.addr).unwrap();
+
+			#[cfg(not(feature = "no-dirty-detection"))]
+			{
 				for p in self.pages.iter() {
 					match &p.snapshot {
 						Snapshot::None => bin::writeval(&mut hasher, 1).unwrap(),
@@ -831,9 +832,10 @@ impl MemoryBlock {
 						Snapshot::Data(d) => { hasher.write(d.slice()).unwrap(); },
 					}
 				}
+			}
+
 			hasher.finalize()[..].to_owned()
 		};
-		}
 
 		Ok(())
 	}