Use the proper add-method to perform the check for max number of children

If adding is done without check it can trigger OOM when fuzzing.

This should fix https://issues.oss-fuzz.com/issues/391709145

poi/src/main/java/org/apache/poi/ddf/UnknownEscherRecord.java

@@ -103,7 +103,7 @@ public final class UnknownEscherRecord extends EscherRecord {
                 bytesWritten += childBytesWritten;
                 offset += childBytesWritten;
                 bytesRemaining -= childBytesWritten;
-                getChildRecords().add( child );
+                addChildRecord( child );
             }
             return bytesWritten;
         }