Fix spurious error from sysrc

When using sysrc to modify a file, the file should be created silently.
However, with the introduction of SVN r335280, an error of "No such file
or directory" would appear despite everything else working as-expected.

The nature of this spurious error is that SVN r335280 did not check if
the file exists first, before trying to fixup the line-endings in the
file just prior to modification.

PR:		bin/240875
Reported by:	Jose Luis Duran
MFC after:	3 days

usr.sbin/bsdconfig/share/sysrc.subr

@@ -560,7 +560,7 @@ f_sysrc_set()
 	#
 	if [ "$not_found" ]; then
 		# Add a newline if missing before appending to the file
-		awk 'BEGIN { wc = 0 } NR == 1 {
+		[ ! -e "$file" ] || awk 'BEGIN { wc = 0 } NR == 1 {
 			(cmd = "wc -l " FILENAME) | getline
 			close(cmd)
 			wc = $1