tail: Initialize the stat buffer used when input is stdin

PR:		266284
Reported by:	Jenkins via delphij
Fixes:		7e11889959 ("tail: Fix -f with stdin")
MFC after:	3 days

usr.bin/tail/tail.c

@@ -273,6 +273,7 @@ main(int argc, char *argv[])
 		} else if (fflag) {
 			file.file_name = fn;
 			file.fp = stdin;
+			file.st = sb;
 			follow(&file, style, off);
 		} else {
 			forward(stdin, fn, style, off, &sb);