exit nonzero upon write failure about zstd

[Cyan4973]

Self note : programs/zstd already detects such situation when input is not stdin, that is :

$ programs/zstd somefilename > /dev/full; echo $?
Error 25 : Write error : cannot write compressed block
25

[Cyan4973]

Jim Meyering :
You just got lucky/unlucky.
The code detects fwrite failure, but not the failure of the write induced by fclose, so if you start with a smaller input or if the error strikes only for the final write, that other usage fails to detect the error, too:
$ echo foo > k && programs/zstd k > x; echo $?
Compressed 4 bytes into 14 bytes ==> 350.00%
0

[Cyan4973]

Fixed into 5b14760
Also : this test has been added to the automated test suite