Restore using NoMoreImageData errors.

#496 has incorrectly started
to return `UnexpectedEof` in a situation when when there is no
`Read`-level EOF.  In particular, this may happen when an `IDAT` of
`fdAT` chunk has been fully decoded, decompressed, and flushed, but
still doesn't contain the expected number of image pixels.  In such a
situation the old `NoMoreImageData` error was appropriate.  In a sense,
this commit is a partial revert and/or refactoring of the earlier commit
b5b0674

This commit fixes a timeout found locally by the `buf_independent`
fuzzer.  The repro case has been saved under
`fuzz/corpus/buf_independent/regressions`.

src/decoder/mod.rs

@@ -702,7 +702,9 @@ impl<R: Read> Reader<R> {
         // Read image data until we have at least one full row (but possibly more than one).
         while self.data_stream.len() - self.current_start < rowlen {
             if self.subframe.consumed_and_flushed {
-                return Err(DecodingError::IoError(ErrorKind::UnexpectedEof.into()));
+                return Err(DecodingError::Format(
+                    FormatErrorInner::NoMoreImageData.into(),
+                ));
             }
 
             // Clear the current buffer before appending more data.
@@ -716,12 +718,9 @@ impl<R: Read> Reader<R> {
 
             match self.decoder.decode_next(&mut self.data_stream)? {
                 Some(Decoded::ImageData) => {}
-                Some(Decoded::ImageDataFlushed) => {
+                Some(Decoded::ImageDataFlushed) | None /* after IEND chunk */ => {
                     self.subframe.consumed_and_flushed = true;
                 }
-                None => {
-                    return Err(DecodingError::IoError(ErrorKind::UnexpectedEof.into()));
-                }
                 _ => (),
             }
         }

src/decoder/stream.rs

@@ -233,6 +233,8 @@ pub(crate) enum FormatErrorInner {
     CorruptFlateStream {
         err: fdeflate::DecompressionError,
     },
+    /// The image data chunk was too short for the expected pixel count.
+    NoMoreImageData,
     /// Bad text encoding
     BadTextEncoding(TextDecodingError),
     /// fdAT shorter than 4 bytes
@@ -333,6 +335,10 @@ impl fmt::Display for FormatError {
             UnknownInterlaceMethod(nr) => write!(fmt, "Unknown interlace method {}.", nr),
             BadSubFrameBounds {} => write!(fmt, "Sub frame is out-of-bounds."),
             InvalidSignature => write!(fmt, "Invalid PNG signature."),
+            NoMoreImageData => write!(
+                fmt,
+                "IDAT or fDAT chunk does not have enough data for image."
+            ),
             CorruptFlateStream { err } => {
                 write!(fmt, "Corrupt deflate stream. ")?;
                 write!(fmt, "{:?}", err)