fix failed write reporting by fwrite in line-buffered mode

when a write error occurred while flushing output due to a newline,
fwrite falsely reported all bytes up to and including the newline as
successfully written. in general, due to buffering such "spurious
success" returns are acceptable for stdio; however for line-buffered
mode it was subtly wrong. errors were still visible via ferror() or as
a short-write return if there was more data past the newline that
should have been written, but since the contract for line-buffered
mode is that everything up through the newline be written out
immediately, a discrepency was observable in the actual file contents.

src/stdio/fwrite.c

@@ -13,8 +13,8 @@ size_t __fwritex(const unsigned char *restrict s, size_t l, FILE *restrict f)
 		/* Match /^(.*\n|)/ */
 		for (i=l; i && s[i-1] != '\n'; i--);
 		if (i) {
-			if (f->write(f, s, i) < i)
-				return i;
+			size_t n = f->write(f, s, i);
+			if (n < i) return n;
 			s += i;
 			l -= i;
 		}