Avoid split_at panic in ScanLines

src/png/scan_lines.rs

@@ -24,7 +24,12 @@ impl<'a> Iterator for ScanLines<'a> {
     type Item = ScanLine<'a>;
     #[inline]
     fn next(&mut self) -> Option<Self::Item> {
-        self.iter.next().map(|(len, pass, num_pixels)| {
+        let (len, pass, num_pixels) = self.iter.next()?;
+        debug_assert!(self.raw_data.len() >= len);
+        debug_assert!(!self.has_filter || len > 1);
+        if self.raw_data.len() < len {
+            return None;
+        }
         let (data, rest) = self.raw_data.split_at(len);
         self.raw_data = rest;
         let (&filter, data) = if self.has_filter {
@@ -32,12 +37,11 @@ impl<'a> Iterator for ScanLines<'a> {
         } else {
             (&0, data)
         };
-            ScanLine {
+        Some(ScanLine {
             filter,
             data,
             pass,
             num_pixels,
-            }
         })
     }
 }