waldump: fix use-after-free in search_directory().

After closedir() dirent->d_name is not valid anymore. As there alerady are a
few places relying on the limited lifetime of pg_waldump, do so here as well,
and just pg_strdup() the string.

The bug was introduced in fc49e24fa6.

Found by UBSan, run locally.

Backpatch: 11-, like fc49e24fa6 itself.

src/bin/pg_waldump/pg_waldump.c

@@ -177,7 +177,7 @@ search_directory(const char *directory, const char *fname)
 			if (IsXLogFileName(xlde->d_name))
 			{
 				fd = open_file_in_directory(directory, xlde->d_name);
-				fname = xlde->d_name;
+				fname = pg_strdup(xlde->d_name);
 				break;
 			}
 		}