pg: fix wcstombs() use

The size (3rd) argument should be ignored if the 1st *dest is NULL, but it seems gcc & glibc headers
are more pedantic now:

ext-utils/pg.c:456:13: error: argument 1 is null but the corresponding size argument 3 value is 2048 [-Werror=nonnull]

Signed-off-by: Karel Zak <kzak@redhat.com>

text-utils/pg.c

@@ -453,7 +453,7 @@ static char *endline_for_mb(unsigned col, char *s)
  ended:
 	*end = L'\0';
 	p = wbuf;
-	if ((pos = wcstombs(NULL, p, READBUF)) == (size_t)-1)
+	if ((pos = wcstombs(NULL, p, 0)) == (size_t)-1)
 		return s + 1;
 	return s + pos;
 }