Initialize variable to avoid a warning in wxCaret unit test

Always initialize the caret pointer in the test added in 88f808e303 (Add
a unit test for wxCaret::Create(), 2020-10-01) to avoid a MSVS warning.

tests/controls/windowtest.cpp

@@ -135,7 +135,7 @@ TEST_CASE_METHOD(WindowTestCase, "Window::Mouse", "[window]")
 #if wxUSE_CARET
     CHECK(!m_window->GetCaret());
 
-    wxCaret* caret;
+    wxCaret* caret = NULL;
 
     // Try creating the caret in two different, but normally equivalent, ways.
     SECTION("Caret 1-step")