Remove assert from wxStringOperationsUtf8::DecIter().

The assert prevented decrementing end() iterator, which is a valid
operation.

Fixes #15566.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74976 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/stringops.h

@@ -79,8 +79,6 @@ struct WXDLLIMPEXP_BASE wxStringOperationsUtf8
     template<typename Iterator>
     static void DecIter(Iterator& i)
     {
-        wxASSERT( IsValidUtf8LeadByte(*i) );
-
         // Non-lead bytes are all in the 0x80..0xBF range (i.e. 10xxxxxx in
         // binary), so we just have to go back until we hit a byte that is
         // either < 0x80 (i.e. 0xxxxxxx in binary) or 0xC0..0xFF (11xxxxxx in