Qt text search crash fix

About 5 weeks ago, commit e4106bb161
changed Qt's searching in text windows to be able to find a match
on the very first line.  It assumed that the first line would be
the current line except when repeating the same search after a
match.  But after a failed search the current line index is -1 and
starting a new search would crash trying to look that line up.

win/Qt/qt_menu.cpp

@@ -1192,6 +1192,8 @@ void NetHackQtTextWindow::Search()
     if (get_a_line) {
         int linecount = lines->count();
         int current = lines->currentRow();
+        if (current == -1)
+            current = 0;
         // when no row is highlighted (selected), start the search
         // on the current row, otherwise start on the row after it
         // [normally means that the very first row is a candidate