Emit InvalidPart when missing an MPU part

src/main/java/org/gaul/s3proxy/S3ErrorCode.java

@@ -49,6 +49,10 @@ enum S3ErrorCode {
             "The specified location constraint is not valid. For" +
             " more information about Regions, see How to Select" +
             " a Region for Your Buckets."),
+    INVALID_PART(HttpServletResponse.SC_BAD_REQUEST,
+            "One or more of the specified parts could not be found." +
+            "  The part may not have been uploaded, or the specified entity" +
+            " tag may not match the part's entity tag."),
     INVALID_REQUEST(HttpServletResponse.SC_BAD_REQUEST, "Bad Request"),
     MALFORMED_X_M_L(HttpServletResponse.SC_BAD_REQUEST,
             "The XML you provided was not well-formed or did not validate" +

src/main/java/org/gaul/s3proxy/S3ProxyHandler.java

@@ -1239,7 +1239,11 @@ final class S3ProxyHandler extends AbstractHandler {
                     parseCompleteMultipartUpload(is).entrySet().iterator();
                     it.hasNext();) {
                 Map.Entry<Integer, String> entry = it.next();
-                long partSize = partsByListing.get(entry.getKey()).partSize();
+                MultipartPart part = partsByListing.get(entry.getKey());
+                if (part == null) {
+                    throw new S3Exception(S3ErrorCode.INVALID_PART);
+                }
+                long partSize = part.partSize();
                 if (partSize < blobStore.getMinimumMultipartPartSize() &&
                         partSize != -1 && it.hasNext()) {
                     throw new S3Exception(S3ErrorCode.ENTITY_TOO_SMALL);