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)

src/msw/debughlp.cpp

@@ -66,7 +66,7 @@ class VarSizedStruct
 public:
     VarSizedStruct()
     {
-        ::ZeroMemory(m_buffer, sizeof(T) + MAX_NAME_LEN);
+        ::ZeroMemory(m_buffer, sizeof(T) + MAX_NAME_LEN*sizeof(TCHAR));
 
         (*this)->SizeOfStruct = sizeof(T);
         (*this)->MaxNameLen = MAX_NAME_LEN;
@@ -87,7 +87,7 @@ private:
     // if we wanted.
     enum { MAX_NAME_LEN = 1024 };
 
-    BYTE m_buffer[sizeof(T) + MAX_NAME_LEN];
+    BYTE m_buffer[sizeof(T) + MAX_NAME_LEN*sizeof(TCHAR)];
 };
 
 } // anonymous namespace