[Rd] Matrix dimnames crash (PR#13361)

[arnima at u.washington.edu]

In Windows XP, the matrix() function crashes the program when 'dimnames' 
is an empty list:

   matrix(1:4, nrow=2, dimnames=list())
   # R has encountered a problem and needs to close ...

This bug is specific to WinXP, as Linux64 handles this situation more 
gracefully:

   matrix(1:4, nrow=2, dimnames=list())
   Error in matrix(1:4, nrow = 2, dimnames = list()) :
     invalid type (environment) for 'dimnames' (must be a vector)

Thanks,
Arni

R 2.8.0-patched on WinXP