split: reset errno prior to getline()

Something else may have set errno, breaking the post-getline() logic
that tries to detect the getline() error.  This was initially noted in
a jail on a system that has HPET, in a jail that does not expose
/dev/hpet0 -- we see an earlier error in libc's vdso bits.

Fixes:	5c053aa3c5 ("split: switch to getline() [...]")

usr.bin/split/split.c

@@ -278,7 +278,7 @@ split2(void)
 		err(EX_NOINPUT, "fdopen");
 
 	/* Process input one line at a time */
-	while ((len = getline(&buf, &bufsize, infp)) > 0) {
+	while ((errno = 0, len = getline(&buf, &bufsize, infp)) > 0) {
 		/* Check if we need to start a new file */
 		if (pflag) {
 			regmatch_t pmatch;