Before the change campaign bonus selection screen had inconsistent
overflow behaviour for difficulty selection:
```
0 1 2 3 4 | available buttons: '-' to decrease
^ | '+' to increase
```
Before the change:
1. If we click '+' 5 times we will end up on difficulty=4 (ok, saturated).
2. If we click '-' 5 times we will end up on difficulty=1 (unexpected, wrap around).
After the change:
1. If we click '+' 5 times we will end up on difficulty=4 (saturated).
2. If we click '-' 5 times we will end up on difficulty=0 (saturated).
The inconsistency happens because `difficulty` variable has `ui8` type
and server uses `difficulty = vstd::abetween(difficulty, 0, 4)` to
implement the saturation.
For large positive values saturation works as expected:
vstd::abetween(difficulty=5, 0, 4) -> 4
For small values it does not:
vstd::abetween(difficulty=-1, 0, 4) -> 4
The change makes client to avoid using negative values.
Noticed use of uninitialized value when built vcmi with -fsanitize=undefined:
```
Established connection with
VCMI 0.99 b310f2e61e (server).
UUID: bab9a90d-7416-4566-8817-e367ffcac7c1
../vcmi-9999/client/lobby/SelectionTab.cpp:138:16:
runtime error: load of value 32717, which is not a valid value for type 'ESortBy'
/usr/lib/gcc/x86_64-pc-linux-gnu/12.0.0/include/g++-v12/tuple:190:4:
runtime error: load of value 32717, which is not a valid value for type 'ESortBy'
/usr/lib/gcc/x86_64-pc-linux-gnu/12.0.0/include/g++-v12/tuple:190:4:
runtime error: load of value 32717, which is not a valid value for type 'ESortBy'
```
Here (before the change) `SelectionTab()` used `generalSortingBy` before
first use:
```
SelectionTab::SelectionTab(ESelectionScreen Type) {
...
if(tabType != ESelectionScreen::campaignList)
{
...
ESortBy criteria = (ESortBy)i;
if(criteria == _name)
criteria = generalSortingBy;
buttonsSortBy.push_back(... std::bind(&SelectionTab::sortBy, this, criteria)));
...
}
...
switch(tabType)
{
case ESelectionScreen::newGame:
generalSortingBy = ESortBy::_name;
....
```
The change moves `generalSortingBy` initialization before first use.
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
