Handle non-standard sBIT headers

Closes #141

src/reduction/alpha.rs

@@ -29,8 +29,11 @@ pub fn reduce_alpha_channel(png: &mut PngData, channels: u8) -> Option<Vec<u8>>
     // sBIT contains information about alpha channel's original depth,
     // and alpha has just been removed
     if let Some(sbit_header) = png.aux_headers.get_mut(b"sBIT") {
-        assert_eq!(sbit_header.len(), channels as usize);
+        // Some programs save the sBIT header as RGB even if the image is RGBA.
-        sbit_header.pop();
+        // Only remove the alpha channel if it's actually there.
+        if sbit_header.len() == channels as usize {
+            sbit_header.pop();
+        }
     }
 
     Some(reduced)

tests/regression.rs

@@ -548,3 +548,15 @@ fn issue_140() {
         BitDepth::Two,
     );
 }
+
+#[test]
+fn issue_141() {
+    test_it_converts(
+        "tests/files/issue-141.png",
+        None,
+        ColorType::RGBA,
+        BitDepth::Eight,
+        ColorType::RGB,
+        BitDepth::Eight,
+    );
+}