fortran: Fix up hash table usage in gfc_trans_use_stmts [PR108451]

fortran: Fix up hash table usage in gfc_trans_use_stmts [PR108451]

The first testcase in the PR (which I haven't included in the patch because
it is unclear to me if it is supposed to be valid or not) ICEs since extra
hash table checking has been added recently.  The problem is that
gfc_trans_use_stmts does
          tree *slot = entry->decls->find_slot_with_hash (rent->use_name, hash,
                                                          INSERT);
          if (*slot == NULL)
and later on doesn't store anything into *slot and continues.  Another spot
a few lines later correctly clears the slot if it decides not to use the
slot, so the following patch does the same.

2023-02-03  Jakub Jelinek  <jakub@redhat.com>

	PR fortran/108451
	* trans-decl.c (gfc_trans_use_stmts): Call clear_slot before
	doing continue.

(cherry picked from commit 76f7f0eddcb7c418d1ec3dea3e2341ca99097301)