fix exception, when DRBD source was not found during DRBD installation

java.lang.NullPointerException
null
java.util.regex.Matcher.appendReplacement(Unknown Source)
java.util.regex.Matcher.replaceAll(Unknown Source)
java.lang.String.replaceAll(Unknown Source)
lcmc.gui.dialog.host.DrbdCommandInst$2.convert(DrbdCommandInst.java:140)
lcmc.utilities.Tools.getDistCommand(Tools.java:920)
lcmc.data.Host.execCommandInBash(Host.java:1180)
lcmc.gui.dialog.host.DrbdCommandInst.installDrbd(DrbdCommandInst.java:115)