FLAC: Stop moving padding blocks

`PADDING` blocks were incorrectly getting moved to the end of the header without the `Last-metadata-block` getting set.

CHANGELOG.md

@@ -11,6 +11,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - This behavior already exists for OGG formats.
 
 ### Fixed
+- **FLAC**: Stop writing invalid `PADDING` blocks ([issue](https://github.com/Serial-ATA/lofty-rs/issues/442)) ([PR](https://github.com/Serial-ATA/lofty-rs/pull/446))
+  - If a `PADDING` block existed in the original file, and it wasn't placed at the end of the header, it would
+    moved without setting the `Last-metadata-block` flag. This would cause decoders to believe that the file was missing
 - **Fuzzing** (Thanks [@qarmin](https://github.com/qarmin)!) ([PR](https://github.com/Serial-ATA/lofty-rs/pull/444)):
   - **MusePack**: Fix panic when tag sizes exceed the stream length ([issue](https://github.com/Serial-ATA/lofty-rs/issues/440))
   - **AAC**: Fix panic when tag sizes exceed the stream length ([issue](https://github.com/Serial-ATA/lofty-rs/issues/439))

lofty/src/flac/write.rs

@@ -61,7 +61,8 @@ where
 
 	let mut cursor = Cursor::new(file_bytes);
 
-	let mut padding = false;
+	// TODO: We need to actually use padding (https://github.com/Serial-ATA/lofty-rs/issues/445)
+	let mut end_padding_exists = false;
 	let mut last_block_info = (
 		stream_info.byte,
 		stream_info.start as usize,
@@ -103,14 +104,20 @@ where
 				tag.vendor = Cow::Owned(vendor_str);
 			},
 			BLOCK_ID_PICTURE => blocks_to_remove.push((start, end)),
-			BLOCK_ID_PADDING => padding = true,
+			BLOCK_ID_PADDING => {
+				if last_block {
+					end_padding_exists = true
+				} else {
+					blocks_to_remove.push((start, end))
+				}
+			},
 			_ => {},
 		}
 	}
 
 	let mut file_bytes = cursor.into_inner();
 
-	if !padding {
+	if !end_padding_exists {
 		if let Some(preferred_padding) = write_options.preferred_padding {
 			log::warn!("File is missing a PADDING block. Adding one");