[Rd] data.frame backwards compatibility (PR#9601)

[v.moreno at ico.scs.es]

Full_Name: Victor Moreno
Version: R2.5.0
OS: windows
Submission from: (NULL) (68.40.63.169)


This may not be a bug, but seems not yet documented. Some data.frames created
with development version 2.5.0, when read in 2.4.1, show error:

Error in dim.data.frame(chip23) : negative length vectors are not allowed

A dump of the data.frame shows as last code:

, row.names = as.integer(c(NA, -7269)), class = "data.frame") 

This seems related to the new feature of data.frames without rownames.