An incomplete fix for the NPE bugs in AbstractJavadocMojo.java

The fix revision 554202 was aimed to remove an NPE bug on the returned value of "getJavadocDirectory()" in the method "getSourcePaths " of the file "/maven/plugins/trunk/maven-javadoc-plugin/src/main/java/org/apache/maven/plugin/javadoc/AbstractJavadocMojo.java" , but it is incomplete.
Since the returned value "getJavadocDirectory()" could be null during the runtime execution, its value should also be null-checked before being dereferenced in other methods.

The buggy code locations the same fix needs to be applied at are as bellows:

Line 2401 of the method "copyJavadocResources";
Line 1505 of the method "getSourcePaths".