(issue1794) Safeguard against layers without extent. Just create a broken layer but don't create a syntax error in a mapfile. The syntax error pulls down the whole user wms while this just creates one broken layer.

SYMBOLSET
SYMBOL
  NAME 'point'
  TYPE ELLIPSE
  POINTS
    1 1
  END
  FILLED TRUE
END
SYMBOL
  NAME "square"
  TYPE VECTOR
  POINTS
    0 0
    0 1
    1 1
    1 0
    0 0
  END
  FILLED TRUE
END
END