Fix an exception in drive_virtual.cpp

[zhongjianhua163]

我很困惑， 为什么第236行在对current_normalized_dir的尾部取值前进行了空字符串的判断，而在第235行中对头部进行取值时却没有判断空字符串？？

我在首次测试中就遇到此处触发的异常。由于DEBUG版在遇到此异常时无法继续，所以我添加了空字符串的判断。

[joncampbell123]

Google translate:

"I'm confused. Why does line 236 check an empty string before taking a value for the current_normalized_dir's tail, but not an empty string when taking the head value in line 235? I encountered an exception triggered here during my first test. Since the DEBUG version cannot continue when encountering this exception, I added an empty string check."

[zhongjianhua163]

Google translate:

"I'm confused. Why does line 236 check an empty string before taking a value for the current_normalized_dir's tail, but not an empty string when taking the head value in line 235? I encountered an exception triggered here during my first test. Since the DEBUG version cannot continue when encountering this exception, I added an empty string check."

“but not an empty string ...” -> "but not check an empty string ..." maybe, im not sure, haha