gnulib/regexec: Fix resource leak

gnulib/regexec: Fix resource leak

In the function merge_state_with_log(), memory is allocated for the variable
next_nodes when creating a union of the variables table_nodes and log_nodes.
However, if next_state->entrance_nodes is NULL, then table_nodes becomes NULL
and we still allocate memory to copy the content of log_nodes. This can cause
a resource leak since we only free the memory for next_nodes if table_nodes
isn't NULL. To prevent this, we need to check that next_state->entrance_nodes
isn't NULL before allocating memory for the union.

This issue has been fixed in the latest version of gnulib and I've backported
this change to maintain consistency.

This issue was found by a Coverity scan of GRUB2 under the CID 473887.

Fixes: CID 473887
Signed-off-by: Alec Brown <alec.r.brown@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>