Put in check to see if stdin exists

- p3dfft.2.3.2/sample/C/*.c about line 61
There is no check wether file stdin exists. This leads to hard to find
segmentations faults, if the user didn't copy stdin from the sample folder 
into the C folder.

A warning for the user would be nice. So I replaced
> fp = fopen("stdin","r");
> ndim = 2;
> fscanf(fp,"%d,%d,%d,%d,%d\n",&nx,&ny,&nz,&ndim,&n);
> fclose(fp);
by
> ndim = 2;
> if((fp=fopen("stdin", "r"))==NULL){
>   printf("Cannot open file. Setting to default nx=ny=nz=128, ndim=2, n=1.
\n");
>   nx=ny=nz=128; n=1;
> }
> else{
>   fscanf(fp,"%d,%d,%d,%d,%d\n",&nx,&ny,&nz,&ndim,&n);
>   fclose(fp);
> }

Original issue reported on code.google.com by d4p...@ucsd.edu on 3 Mar 2010 at 1:23

jasonlarkin / p3dfft

Put in check to see if stdin exists #19