Fixed heap corruption bug.

The RDP server can send both compressed and uncompressed bitmap
rectangles at same time. Attempting to uncompress uncompressed
rectangles with bitmap_decompress() seems to lead to a crash or
execution block.

Now bitmap_decompress() is only called when the rectangle is really
compressed.

Signed-off-by: Janne Hakonen <janne.hakonen@oss.tieto.com>