fix: correct null-check variable after db_umem registration in ibdevx

[Doondi-Ashlesh]

Summary

Fixes a copy-paste bug in the ibdevx QP initialization code reported in #74.

After registering db_umem, the null-check macro incorrectly checks wq_umem instead of db_umem:

// Before (incorrect):
db_umem = mlx5dv_devx_umem_reg(context, dbr_buf, NVSHMEMT_IBDEVX_DBSIZE, 0);
NVSHMEMI_NULL_ERROR_JMP(wq_umem, ...);  // checks wrong variable

// After (correct):
db_umem = mlx5dv_devx_umem_reg(context, dbr_buf, NVSHMEMT_IBDEVX_DBSIZE, 0);
NVSHMEMI_NULL_ERROR_JMP(db_umem, ...);  // checks the just-registered variable

If db_umem registration fails and returns NULL, the error would go undetected (since wq_umem was already successfully registered and is non-null), leading to a later null dereference at line 447 when db_umem->umem_id is accessed.

Fixes #74

[Doondi-Ashlesh]

Hi, just checking in on this PR, it should be ready to merge once reviewed.
Happy to make changes if anything needs to be updated. Thank you.