Fix strftime() buffer overflow (#362)

If the formatted string is longer than 17 characters the string won't
be null terminated, causing gibberish to be printed on the status line
and possibly worse.

This occurs with my current locale in which a date is printed as such:

  2019-okt.-15 19:26

Which is 18 characters.

src/nnn.c

@@ -2530,11 +2530,12 @@ static void printent(const struct entry *ent, int sel, uint namecols)
 
 static void printent_long(const struct entry *ent, int sel, uint namecols)
 {
-	char timebuf[18], permbuf[4], ind1 = '\0', ind2[] = "\0\0";
+	char timebuf[24], permbuf[4], ind1 = '\0', ind2[] = "\0\0";
 	const char cp = (ent->flags & FILE_SELECTED) ? '+' : ' ';
 
 	/* Timestamp */
-	strftime(timebuf, 18, "%F %R", localtime(&ent->t));
+	strftime(timebuf, sizeof(timebuf), "%F %R", localtime(&ent->t));
+	timebuf[sizeof(timebuf)-1] = '\0';
 
 	/* Permissions */
 	permbuf[0] = '0' + ((ent->mode >> 6) & 7);
@@ -3439,7 +3440,7 @@ static void redraw(char *path)
 	int ncols = (xcols <= PATH_MAX) ? xcols : PATH_MAX;
 	int lastln = xlines, onscreen = xlines - 4;
 	int i, attrs;
-	char buf[18];
+	char buf[24];
 	char c;
 	char *ptr = path, *base;
 
@@ -3579,7 +3580,8 @@ static void redraw(char *path)
 			base = unescape(pent->name, NAME_MAX, NULL);
 
 			/* Timestamp */
-			strftime(buf, 18, "%Y-%b-%d %R", localtime(&pent->t));
+			strftime(buf, sizeof(buf), "%Y-%b-%d %R", localtime(&pent->t));
+			buf[sizeof(buf)-1] = '\0';
 
 			mvprintw(lastln, 0, "%d/%d (%d) %s%s %s %s %s [%s]",
 				 cur + 1, ndents, nselected, sort, buf,