Don't crash on trailing '%' in wxDateTime::Format(). See #17931

src/common/datetimefmt.cpp

@@ -334,6 +334,7 @@ wxString wxDateTime::Format(const wxString& formatp, const TimeZone& tz) const
     time_t time = GetTicks();
 
     bool canUseStrftime = time != (time_t)-1;
+    bool isPercent = false;
 
     // We also can't use strftime() if we use non standard specifier: either
     // our own extension "%l" or one of "%g", "%G", "%V", "%z" which are POSIX
@@ -342,11 +343,15 @@ wxString wxDateTime::Format(const wxString& formatp, const TimeZone& tz) const
           canUseStrftime && p != format.end();
           ++p )
     {
-        if ( *p != '%' )
+        if (!isPercent)
+        {
+            isPercent = *p == '%';
             continue;
+        }
+        isPercent = false;
 
         // set the default format
-        switch ( (*++p).GetValue() )
+        switch ( (*p).GetValue() )
         {
             case 'l':
 #ifdef __WINDOWS__

tests/datetime/datetimetest.cpp

@@ -822,6 +822,8 @@ void DateTimeTestCase::TestTimeFormat()
         }
     }
 
+    CPPUNIT_ASSERT(wxDateTime::Now().Format("%") == "%");
+
     wxDateTime dt;
 
 #if 0