Sanity checks (fuzzer)

I ran a stupid fuzzer on all the plugins in the repo and some plugins needs more sanity checks.

- RAS: fixed palette reading on corrupted files
- RGB: improved error detection on datastream

This patch improves the reading speed of some corrupted files and limit the maximum memory allocation of RAS palette.

src/imageformats/rgb.cpp

@@ -360,11 +360,15 @@ bool SGIImage::readImage(QImage &img)
         }
 
         _lengthtab = new quint32[_numrows];
-        for (l = 0; l < _numrows; l++) {
+        for (l = 0; !_stream.atEnd() && l < _numrows; l++) {
             _stream >> _lengthtab[l];
         }
     }
 
+    if (_stream.status() != QDataStream::Ok) {
+        return false;
+    }
+
     _data = _dev->readAll();
 
     // sanity check