Fix MCU reference in odd decoding order

This addresses a potential panic due to index errors. When a component
is finished then it can be put into a queue. This references the MCU row
coefficients, some structure filled for each _finished_ component.

However, the list for these MCU row coefficients is allocated lazily.
Previously we would fill it with an instance for each _finished_
component but then used the index of the component (in the list of _all_
components) to access it. This could cause wrong indices or a
past-the-end index and subsequent panic.

Author: 197g

src/decoder.rs

@@ -582,12 +582,12 @@ impl<R: Read> Decoder<R> {
         let mut mcus_left_until_restart = self.restart_interval;
         let mut expected_rst_num = 0;
         let mut eob_run = 0;
-        let mut mcu_row_coefficients = Vec::with_capacity(components.len());
+        let mut mcu_row_coefficients = vec![vec![]; components.len()];
 
         if !is_progressive {
-            for (_, component) in components.iter().enumerate().filter(|&(i, _)| finished[i]) {
+            for (i, component) in components.iter().enumerate().filter(|&(i, _)| finished[i]) {
                 let coefficients_per_mcu_row = component.block_size.width as usize * component.vertical_sampling_factor as usize * 64;
-                mcu_row_coefficients.push(vec![0i16; coefficients_per_mcu_row]);
+                mcu_row_coefficients[i] = vec![0i16; coefficients_per_mcu_row];
             }
         }