[gfortran] Fix PR 15754: Make P = NULL() issue a warning

[Paul Brook]

> 2004-07-11  Tobias Schlueter   <tobias.schlueter@physik.uni-muenchen.de>
>
> 	PR fortran/15754
> 	* expr.c (gfc_check_assign): Print ranks if incompatible. Issue
> 	warning if assigning NULL().

Ok.

Paul