Fix NPE in UnnecessaryLocalBeforeReturnRule

- Fixes #1804

pmd-java/src/main/java/net/sourceforge/pmd/lang/java/rule/codestyle/UnnecessaryLocalBeforeReturnRule.java

@@ -136,7 +136,7 @@ public class UnnecessaryLocalBeforeReturnRule extends AbstractJavaRule {
                                 final ASTBlockStatement location = occ.getLocation().getFirstParentOfType(ASTBlockStatement.class);
                                 
                                 // Is it used after initializing our "unnecessary" local but before the return statement?
-                                if (isAfter(location, initializerStmt) && isAfter(rtnStmt, location)) {
+                                if (location != null && isAfter(location, initializerStmt) && isAfter(rtnStmt, location)) {
                                     return true;
                                 }
                             }

pmd-java/src/test/resources/net/sourceforge/pmd/lang/java/rule/codestyle/xml/UnnecessaryLocalBeforeReturn.xml

@@ -244,6 +244,23 @@ public class UnnecessaryLocalBeforeReturnFP {
         	.make(i);
         return o; // true positive
     }  
+}
+        ]]></code>
+    </test-code>
+    
+    <test-code>
+        <description>#1804 [java] NPE with fields</description>
+        <expected-problems>1</expected-problems>
+        <code><![CDATA[
+public class ObjectCreator {
+
+    private static final String A = ""; 
+    private static final String B = "" + A; // the existence of this line causes the NPE.
+
+    public Object create() {
+        final Object o = new Object(A);
+        return o;
+    }   
 }
         ]]></code>
     </test-code>