This fixes VirglStagingMgr tests that tries to access a struct member of
a structure that is NULL.
This has been detected using Undefined Behaviour Sanitizer.
```
Running main() from ../src/gtest/src/gtest_main.cc
[==========] Running 9 tests from 2 test suites.
[----------] Global test environment set-up.
[----------] 7 tests from VirglStagingMgr
[ RUN ] VirglStagingMgr.non_fitting_allocation_reallocates_resource
stderr:
../src/gallium/drivers/virgl/tests/virgl_staging_mgr_test.cpp:72:22: runtime error: member access within null pointer of type 'struct virgl_hw_res'
```
Reviewed-by: Eric Engestrom <eric@igalia.com>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30880>