lra: set insn_code_data to NULL when freeing

lra: set insn_code_data to NULL when freeing

libgccjit's test-threads.c repeatedly compiles and runs numerous tests,
each in a separate thread.

Attempting to add an empty test that generates no code leads to a
double-free ICE within that thread, within lra.c's
finish_insn_code_data_once.

The root cause is that the insn_code_data array is cleared in
init_insn_code_data_once, but this is only called the first time
a cgraph_node is expanded [1], whereas the "loop-over-all-elements
and free them" is unconditionally called in finalize [2].  Hence
if there are no functions:
* the array is not re-initialized for the empty context
* when finish_insn_code_data_once is called for the empty context
it still contains the freed pointers from the previous context
that held the jit mutex, and hence the free is a double-free.

This patch sets the pointers to NULL after freeing them, fixing
the ICE.

[1]
init_insn_code_data_once is called via
  lra_init_once called by
    ira_init_once called by
      initialize_rtl, via:
         if (!rtl_initialized)
           ira_init_once ();
        called by init_function_start
           called by cgraph_node::expand

[2]:
finish_insn_code_data_once is called by:
  lra_finish_once called by
    finalize

gcc/ChangeLog:
	* lra.c (finish_insn_code_data_once): Set the array elements
	to NULL after freeing them.

gcc/testsuite/ChangeLog:
	* jit.dg/all-non-failing-tests.h: Add test-empty.c