Control-Z bug fix

* Fix for seg fault when pressing ctrl-Z immediately after
    double Escape

src/widgets/Fl_Text_Buffer_mod_1_1.cxx

@@ -273,6 +273,7 @@ void Fl_Text_Buffer_mod::replace( int start, int end, const char *s ) {
   // Range check...
   if (!s) return;
   if (start < 0) start = 0;
+  if (end < 0) end = 0;
   if (end > mLength) end = mLength;
 
   call_predelete_callbacks( start, end-start );

src/widgets/Fl_Text_Buffer_mod_1_3.cxx

@@ -335,6 +335,8 @@ void Fl_Text_Buffer_mod::replace(int start, int end, const char *text)
     return;
   if (start < 0)
     start = 0;
+  if (end < 0)
+    end = 0;
   if (end > mLength)
     end = mLength;