Fix memory leak problem in ufdt_node_pool_destruct

ufdt_node_pool could delay memory block freeing to enhance
performance. The left memory blocks should be freed in
ufdt_node_pool_destruct().

The patch fix the problem that a wrong code block to free
left memory blocks.

Bug: 62474732
Test: tests/run_stress_tests.sh with unmark VALGRIND in
      apply_overlay.sh
Change-Id: I5fd2f51a342501ad5bc27b26d343893a822db6b5