Fix assertion in PosixResources

Author: msimacek

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/runtime/PosixResources.java

@@ -264,10 +264,11 @@ private void dupFD(int fd1, int fd2) {
         ChannelWrapper channelWrapper = files.getOrDefault(fd1, null);
         String path = filePaths.get(fd1);
         if (channelWrapper != null) {
-            assert path != null;
             channelWrapper.cnt += 1;
             files.put(fd2, channelWrapper);
-            filePaths.put(fd2, path);
+            if (path != null) {
+                filePaths.put(fd2, path);
+            }
         }
     }