Manager, computing prefs dialog: Fix bug where the "In use means..." value mysteriously showed up in the "suspend when non-BOINC..." field.

[davidpanderson]

The problem was that wxWidgets' ToDouble(&x) returns an error (which we weren't checking for)
and doesn't modify x if the string is empty.
So we were getting the value from a previous field.

Solution: set x to zero before each such call.

Fixes the bug reported in #5266

[AenBleidd]

@davidpanderson, please fix trailing whitespaces issue:

Found files with errors:
./clientgui/DlgAdvPreferences.cpp:573
./clientgui/DlgAdvPreferences.cpp:645
To fix errors run locally:
	python3 ci_tools/trailing_whitespaces_check.py . --fix

[Vulpine05]

I tested this and it looks like it fixed the problem. Thanks!