DICOM writer: don't throw NPE if close was already called

Shouldn't affect bfconvert, but does affect ImageJ export which calls
close() twice.

Author: melissalinkert

Diff for: components/formats-bsd/src/loci/formats/out/DicomWriter.java

@@ -1383,6 +1383,11 @@ public void close() throws IOException {
         for (int res=0; res<resolutionCount; res++) {
           resolution = res;
           openFile(pyramid, resolution);
+
+          if (out == null) {
+            // already closed
+            continue;
+          }
           int resolutionIndex = getIndex(pyramid, resolution);
 
           out.seek(out.length());