mxge(4): avoid clang 21 warning in NO-IP configuration

Building the LINT-NOIP kernel on amd64 with clang 21 results in a
-Werror warning similar to:

    sys/dev/mxge/if_mxge.c:1846:44: error: variable 'sum' is uninitialized when passed as a const pointer argument here [-Werror,-Wuninitialized-const-pointer]
     1846 |                     cksum_offset, sizeof(sum), (caddr_t)&sum);
          |                                                          ^~~

Indeed, if both `INET` and `INET6` are undefined, `sum` is never
initialized. Initialize it to zero to silence the warning.

Reviewed by:	jhibbits
MFC after:	3 days
Differential Revision: https://reviews.freebsd.org/D54730

1 files changed, 1 insertions, 1 deletions

diff --git a/sys/dev/mxge/if_mxge.c b/sys/dev/mxge/if_mxge.c
index f36f41d53b40..ca2267098c4f 100644
--- a/sys/dev/mxge/if_mxge.c
+++ b/sys/dev/mxge/if_mxge.c
@@ -1804,7 +1804,7 @@ mxge_encap_tso(struct mxge_slice_state *ss, struct mbuf *m,
 	uint32_t low, high_swapped;
 	int len, seglen, cum_len, cum_len_next;
 	int next_is_first, chop, cnt, rdma_count, small;
-	uint16_t pseudo_hdr_offset, cksum_offset, mss, sum;
+	uint16_t pseudo_hdr_offset, cksum_offset, mss, sum = 0;
 	uint8_t flags, flags_next;
 	static int once;