[][kernel][common][hnat][Fix issues detected by Coverity scan]

[Description]
Fix issues detected by Coverity scan
1. mtk_hnat_ipv4_nf_pre_routing:
	- dereference before null check(skb null check)

[Release-log]
N/A


Change-Id: Idd2e8596b525eda76e0aa63484e59ca40292ea96
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/8857331
1 file changed