Fix setting archiver destination file to a one in the working dir causes NPE

Fixes #94

Author: plamentotev

src/main/java/org/codehaus/plexus/archiver/AbstractArchiver.java

@@ -669,7 +669,7 @@ public void setDestFile( final File destFile )
     {
         this.destFile = destFile;
 
-        if ( destFile != null )
+        if ( destFile != null && destFile.getParentFile() != null )
         {
             destFile.getParentFile().mkdirs();
         }

src/test/java/org/codehaus/plexus/archiver/AbstractArchiverTest.java

@@ -1,5 +1,6 @@
 package org.codehaus.plexus.archiver;
 
+import java.io.File;
 import java.io.IOException;
 import junit.framework.TestCase;
 
@@ -89,4 +90,8 @@ public void testOverridesCanBeReset()
         assertEquals( -1, archiver.getOverrideDirectoryMode() );
     }
 
+    public void testSetDestFileInTheWorkingDir() {
+        archiver.setDestFile( new File( "archive" ) );
+    }
+
 }