fix npe

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1917138 13f79535-47bb-0310-9956-ffa450edef68

poi-ooxml/src/main/java/org/apache/poi/openxml4j/opc/PackagingURIHelper.java

@@ -165,7 +165,9 @@ public final class PackagingURIHelper {
         if (partUri == null)
             throw new IllegalArgumentException("partUri");
 
-        return partUri.getPath().matches(
+        final String path = partUri.getPath();
+
+        return path != null && path.matches(
                 ".*" + RELATIONSHIP_PART_SEGMENT_NAME + ".*"
                         + RELATIONSHIP_PART_EXTENSION_NAME + "$");
     }