Hi,

After updating to the latest CVS version this morning I encountered
problems sending signed mail. I got messages like: 

  Warning, attempt to set invalid signencrypt-style

in the minibuffer. I traced this back to the mml-generate-mime-1
function in mml.el. There mml-signencrypt-style was called with a null
argument, because my buffer only contained a sign component, and not
an encrypt component. The following patch seems to fix that, but I'm
not sure if this is the proper fix.

Hans