tga: adjust colormap lookup to match tga 2.0 spec (#2608)

Author: mstoeckl

src/codecs/tga/decoder.rs

@@ -20,8 +20,7 @@ struct ColorMap {
 impl ColorMap {
     /// Get one entry from the color map
     pub(crate) fn get(&self, index: usize) -> Option<&[u8]> {
-        // TODO: Should we actually be *subtracting* start_offset from the index here?
-        let entry = self.start_offset + self.entry_size * index;
+        let entry = self.entry_size * index.checked_sub(self.start_offset)?;
         self.bytes.get(entry..entry + self.entry_size)
     }
 }