riched20: Check for NULL in fnTextSrv_TxSetText and add test.

dlls/riched20/tests/txtsrv.c

@@ -681,6 +681,16 @@ static void test_TxSetText(void)
     ok(memcmp(rettext,settext,SysStringByteLen(rettext)) == 0,
                  "String returned differs\n");
 
+    /* Null-pointer should behave the same as empty-string */
+
+    hres = ITextServices_TxSetText(txtserv, 0);
+    ok(hres == S_OK, "ITextServices_TxSetText failed (result = %x)\n", hres);
+
+    hres = ITextServices_TxGetText(txtserv, &rettext);
+    ok(hres == S_OK, "ITextServices_TxGetText failed (result = %x)\n", hres);
+    ok(SysStringLen(rettext) == 0,
+                 "String returned of wrong length (expected 0, got %d)\n", SysStringLen(rettext));
+
     SysFreeString(rettext);
     ITextServices_Release(txtserv);
     ITextHost_Release(host);

dlls/riched20/txtsrv.c

@@ -286,10 +286,9 @@ DECLSPEC_HIDDEN HRESULT WINAPI fnTextSrv_TxSetText(ITextServices *iface, LPCWSTR
    ME_Cursor cursor;
 
    ME_SetCursorToStart(This->editor, &cursor);
-   ME_InternalDeleteText(This->editor, &cursor,
-                         ME_GetTextLength(This->editor), FALSE);
-   ME_InsertTextFromCursor(This->editor, 0, pszText, -1,
-                           This->editor->pBuffer->pDefaultStyle);
+   ME_InternalDeleteText(This->editor, &cursor, ME_GetTextLength(This->editor), FALSE);
+   if(pszText)
+       ME_InsertTextFromCursor(This->editor, 0, pszText, -1, This->editor->pBuffer->pDefaultStyle);
    ME_SetSelection(This->editor, 0, 0);
    This->editor->nModifyStep = 0;
    OleFlushClipboard();