History_Manager: Move static variable to outside of fucntion

[ma8ma]

変数のスコープ範囲を減らすことができるとcppcheck 2.8に指摘されたため静的変数を関数の外に移動します。

style: The scope of the variable 'nullitems' can be reduced. Warning: Be careful when fixing this message, especially when there are inner loops. Here is an example where cppcheck will write that the scope for 'i' can be reduced:

void f(int x)
{
    int i = 0;
    if (x) {
        // it's safe to move 'int i = 0;' here
        for (int n = 0; n < 10; ++n) {
            // it is possible but not safe to move 'int i = 0;' here
            do_something(&i);
        }
    }
}

When you see this message it is always safe to reduce the variable scope 1 level. [variableScope]

src/history/historymanager.cpp:467:44: style: The scope of the variable 'nullitems' can be reduced. (snip) [variableScope]
    static std::vector< ViewHistoryItem* > nullitems;
                                           ^
src/history/historymanager.cpp:478:44: style: The scope of the variable 'nullitems' can be reduced. (snip) [variableScope]
    static std::vector< ViewHistoryItem* > nullitems;
                                           ^

関連のpull request: #988