stop leaking on the Lua stack

..even if at the expense of leaking on the heap. Because the Lua stack
has very limited space (~20 slots). When it overflows, we segfault.

src/kilo.c

@@ -1080,6 +1080,7 @@ static int Quit = 0;
 static int Back_to_big_picture = 0;
 extern void save_editor_state(int rowoff, int coloff, int cy, int cx);
 static void editorProcessKeypress(lua_State* L) {
+    lua_settop(L, 0);  /* big hammer to avoid overflowing the stack */
     int c = getch();
 //?     mvprintw(LINES-3, 60, "key: %d\n", c);
 //?     getch();

src/lua.c

@@ -901,10 +901,10 @@ int restore_editor_view (lua_State *L) {
     status = load_editor_buffer_to_current_definition_in_image(L);
     if (status == 0 || lua_isnil(L, -1))
       break;
-    Previous_error = lua_tostring(L, -1);
+    Previous_error = strdup(lua_tostring(L, -1));  /* memory leak */
     if (Previous_error == NULL) Previous_error = "(error object is not a string)";
-    back_to_big_picture = resumeEdit(L);
     lua_pop(L, 1);
+    back_to_big_picture = resumeEdit(L);
   }
   return back_to_big_picture;
 }