bug(dd): when the src is /dev/null it allocates memory even must not for that case

[HackingRepo]

Hi, uutils mainteners

we confirm that when the stressing the du utility, here is how

relunsec@relunsec:~/software/coreutils/target/debug$ ./dd obs=11777777 ibs=1111 bs=7177118117 cbs=8818181111 if=/dev/null of=a
^C

uu dd just allocates memory indefenitevly however since the src is /dev/null, uu dd must not for /dev/null, gnu just says 0 bytes copied

relunsec@relunsec:~/software/coreutils/target/debug$ gnudd obs=11777777 ibs=1111 bs=7177118117 cbs=8818181111 if=/dev/null of=a
0+0 records in
0+0 records out
0 bytes copied, 8.8746e-05 s, 0.0 kB/s
relunsec@relunsec:~/software/coreutils/target/debug$