progerror-tempo fixed

[Erik Sandberg]

This is fixed in 2.5.11.

\version "2.5.1"

\header { texidoc = "
The note head doesn't appear in the \tempo command.  Instead, there are
nasty `programming error' warnings.

Also, lilypond --preview crashes on this file (so my guess is that the 
notehead just is terribly displaced)
" }

{
  \tempo 2=92
  c'
}