Fix buffer overflow in wxMSW stack walking code

VarSizedStruct buffer had a too small size in Unicode build as it forgot
to multiply the name length by sizeof(TCHAR), resulting in overwriting
memory on the stack after it when calling SymFromAddrW().

Closes #18127.

docs/changes.txt

@@ -90,6 +90,7 @@ wxMSW:
 - Fix hang after clearing wxTAB_TRAVERSAL style on a window with children.
 - Fix handling of AUX2 mouse button events (Trylz).
 - Fix saving/restoring window position for maximized windows.
+- Fix stack corruption when using wxStackWalker (srfisk).
 
 
 3.1.1: (released 2018-02-19)