Set startinpos before calling the error handler.

Objects/unicodeobject.c

@@ -1507,6 +1507,7 @@ PyObject *PyUnicode_DecodeUTF7(const char *s,
             }
         }
         else if (SPECIAL(ch,0,0)) {
+            startinpos = s-starts;
             errmsg = "unexpected special character";
             s++;
             goto utf7Error;