The main argument is that it is impossible to determine if %n evaluated or not
when snprintf return 0, because it can happens for both n == 0 and n == 1.
Although EOF here is good indication of the end of process, if n is
decreased in the loop...
Since it is already supposed in many places that EOF *is* negative, f.e.
from Single Unix specs for snprintf
"return ... a negative value if an output error was encountered"
this not makes situation worse.
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