Ensure VarBstrCat handles null parms as per windows and actually does

dlls/oleaut32/variant.c

@@ -4916,18 +4916,22 @@ HRESULT WINAPI VarBstrCmp(BSTR left, BSTR right, LCID lcid, DWORD flags)
 HRESULT WINAPI VarBstrCat(BSTR left, BSTR right, BSTR *out)
 {
     BSTR result;
+    int size = 0;
 
     TRACE("( %s %s %p )\n", debugstr_w(left), debugstr_w(right), out);
 
-    if( (!left) || (!right) || (!out) )
-        return 0;
-
-    result = SysAllocStringLen(left, lstrlenW(left)+lstrlenW(right));
-    lstrcatW(result,right);
+    /* On Windows, NULL parms are still handled (as empty strings) */
+    if (left)  size=size + lstrlenW(left);
+    if (right) size=size + lstrlenW(right);
 
+    if (out) {
+        result = SysAllocStringLen(NULL, size);
         *out = result;
-
-    return 1;
+        if (left) lstrcatW(result,left);
+        if (right) lstrcatW(result,right);
+        TRACE("result = %s, [%p]\n", debugstr_w(result), result);
+    }
+    return S_OK;
 }
 
 /**********************************************************************
@@ -5529,5 +5533,3 @@ HRESULT WINAPI VarFormat(LPVARIANT varIn, LPOLESTR format,
     TRACE("result: '%s'\n", debugstr_w(*pbstrOut));
     return rc;
 }
- 
-