erofs-utils: fix -Ededupe crash without fragments enabled

erofs-utils: fix -Ededupe crash without fragments enabled

The root cause is the same as commit d9baceba7026 ("erofs-utils:
fix -Ededupe crash without fragments enabled").

In fact, the fragment manager should be reworked in the next major
erofs-utils version to avoid those dirty hacks.

Fixes: fc880e31b7c7 ("erofs-utils: mkfs: minor cleanup & rearrangement")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250102113418.3246744-1-hsiangkao@linux.alibaba.com