dd: should error on reading stdin when iflag=directory option is given

GNU coreutils dd 9.4.113-c01ee

$ : | dd iflag=directory
dd: setting flags for 'standard input': Not a directory

uutils dd:

$ : | ./target/debug/dd iflag=directory
0+0 records in
0+0 records out
0 bytes copied, 2.37402e-04 s, 0.0 B/s

Test case found in GNU test suite tests/dd/misc.sh.

Edit: tested on Ubuntu 20.04, where the iflag=directory option is allowed.

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