Dom: Fix accessing first character for std::string

[ma8ma]

std::stringの変数について冗長または範囲外なアクセスの可能性をcppcheckに警告されたため条件文を整理して修正します。

cppcheckのレポート

src/xml/dom.cpp:133:41: warning: Either the condition 'open_tag.empty()' is redundant or expression 'open_tag[0]' cause access out of bounds. [containerOutOfBounds]
            bool is_alpha = ( ( open_tag[0] >= 'A' && open_tag[0] <= 'Z' ) || ( open_tag[0] >= 'a' && open_tag[0] <= 'z' ) );
                                        ^
src/xml/dom.cpp:137:31: note: Assuming that condition 'open_tag.empty()' is not redundant
            if( open_tag.empty() || ! is_alpha ) continue;
                              ^
src/xml/dom.cpp:133:41: note: Access out of bounds
            bool is_alpha = ( ( open_tag[0] >= 'A' && open_tag[0] <= 'Z' ) || ( open_tag[0] >= 'a' && open_tag[0] <= 'z' ) );
                                        ^