Fix uninitialized variable

[kronbichler]

I saw this uninitialized variable when running a relaxation preconditioner with valgrind, and I believe it will also fix the error reported here:
https://cdash.dealii.org/viewTest.php?onlyfailed&buildid=2516

Related to #16747.

[kronbichler]

With this change, the test is now failing in the first three lines,

2,4c2,4
< DEAL::OK!
< DEAL::OK!
< DEAL::OK!
---
> DEAL::ERROR!
> DEAL::ERROR!
> DEAL::ERROR!

This corresponds to the relaxation parameter 0.0, i.e., automatic determination of eigenvalues, running for 1, 2, 3 sweeps. However, the test is designed in such a way that it is really hard to see where exactly we are failing now. @peterrum would you mind looking into the tests as to where they fail? I think this just succeeded by chance before, depending on the spurious value of uninitialized variable. In any case, it would also be desirable if the test could print more precisely where something fails, rather than just accumulate errors in a spaghetti manner over 4 different metrics and 5 different test cases within each variant without printing specific information here:

dealii/tests/lac/precondition_relaxation_01.cc

Lines 346 to 360 in 3ccc91f

	if (std::all_of(results.begin(), results.end(), [&](const auto &a) {
	if (std::abs(std::get<0>(a) - std::get<0>(results[0])) > 1e-6)
	return false;
	if (std::abs(std::get<1>(a) - std::get<1>(results[0])) > 1e-6)
	return false;
	if (std::abs(std::get<2>(a) - std::get<2>(results[0])) > 1e-6)
	return false;
	if (std::abs(std::get<3>(a) - std::get<3>(results[0])) > 1e-6)
	return false;
	return true;
	}))

[peterrum]

I will do.

[kronbichler]

Thanks for the help!

[peterrum]

@kronbichler I have fixed the test and added some asserts.

[kronbichler]

Thank you @peterrum, this looks much better now!

[tamiko]

Let's make this output more than 10 digits. Otherwise, numdiff might trigger accidentally.

[kronbichler]

Good idea, I adjusted the test and its output.