Fix using invalid string index in wxIsAbsolutePath()

Check if the path is at least two characters long before accessing its
second character.

Add test cases for wxIsAbsolutePath() on MS Windows.

Closes https://github.com/wxWidgets/wxWidgets/pull/2262

tests/file/filefn.cpp

@@ -525,6 +525,12 @@ void FileFunctionsTestCase::IsAbsolutePath()
     CPPUNIT_ASSERT( filename.MakeAbsolute() );
     // wxFileName::GetFullPath returns absolute path
     CPPUNIT_ASSERT_MESSAGE( msg, wxIsAbsolutePath(filename.GetFullPath()));
+
+#ifdef __WINDOWS__
+    CPPUNIT_ASSERT( wxIsAbsolutePath("\\"));
+    CPPUNIT_ASSERT( wxIsAbsolutePath("c:"));
+    CPPUNIT_ASSERT( !wxIsAbsolutePath("c"));
+#endif
 }
 
 void FileFunctionsTestCase::PathOnly()