Fix StringIndexOutOfBoundsException in SelectionUtil.getSelectionText()

```java
Exception in thread "AWT-EventQueue-0" java.lang.StringIndexOutOfBoundsException: String index out of range: -5
	at java.lang.String.substring(String.java:1911)
	at com.jediterm.terminal.model.SelectionUtil.getSelectionText(SelectionUtil.java:69)
	at com.jediterm.terminal.ui.TerminalPanel.copySelection(TerminalPanel.java:359)
	at com.jediterm.terminal.ui.TerminalPanel.handleCopy(TerminalPanel.java:1187)
	at com.jediterm.terminal.ui.TerminalPanel.access$16(TerminalPanel.java:1184)
	at com.jediterm.terminal.ui.TerminalPanel$2.mouseDragged(TerminalPanel.java:156)
	at java.awt.AWTEventMulticaster.mouseDragged(AWTEventMulticaster.java:320)
```