ngctl: Fix getline loop

I misremembered when I wrote this code: getline() returns -1 on EOF, not
zero, so the loop condition and the error check are both incorrect
(though in practice getline() will never return 0).

MFC after:	3 days
Fixes:		3cbdcabf71 ("ngctl: Modernize code somewhat")
Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D55487

usr.sbin/ngctl/main.c

@@ -243,7 +243,7 @@ ReadFile(FILE *fp)
 	unsigned int lineno = 0;
 	int rtn = CMDRTN_OK;
 
-	while ((len = getline(&line, &sz, fp)) > 0) {
+	while ((len = getline(&line, &sz, fp)) >= 0) {
 		lineno++;
 		if (*line == '#')
 			continue;
@@ -252,7 +252,7 @@ ReadFile(FILE *fp)
 			break;
 		}
 	}
-	if (len < 0)
+	if (ferror(fp))
 		rtn = CMDRTN_ERROR;
 	free(line);
 	return (rtn);