fix exceptions during "back" button in DRBD wizard

java.lang.NumberFormatException: For input string: ""
For input string: ""
java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
java.lang.Integer.parseInt(Integer.java:504)
java.lang.Integer.parseInt(Integer.java:527)
lcmc.gui.resources.DrbdResourceInfo.addProxyPorts(DrbdResourceInfo.java:1538)

or

java.lang.NumberFormatException: For input string: ""
For input string: ""
java.lang.NumberFormatException.forInputString(Unknown Source)
java.lang.Integer.parseInt(Unknown Source)
java.lang.Integer.parseInt(Unknown Source)
lcmc.gui.resources.DrbdResourceInfo.addHostAddresses(DrbdResourceInfo.java:1396)