fix assumption in fputs that fwrite returning 0 implies an error

fix assumption in fputs that fwrite returning 0 implies an error

internally, the idiom of passing nmemb=1 to fwrite and interpreting
the return value of fwrite (which is necessarily 0 or 1) as
failure/success is fairly widely used. this is not correct, however,
when the size argument is unknown and may be zero, since C requires
fwrite to return 0 in that special case. previously fwrite always
returned nmemb on success, but this was changed for conformance with
ISO C by commit 500c6886c654fd45e4926990fee2c61d816be197.