Fix bugs in DisplayEvent() and GetEventCategory() from EventVwr

Two fixes:

• DisplayEvent(): ListView_GetItemText expects the number of characters as the last param. However it is wrongly calculated.

• GetEventCategory(): Fix the logic in the if..else case:
  a)The else was using a NULL buffer as wcscpy source, which could lead to a memory error. However this part of code is never reached because
  b)The if...else is just executed if FormatMessageW!=0. Since FormatMessageW returns the number of characters from the buffer, checking if the buffer is NULL is unneeded.