ltl_to_tgba_fm: fix a memory leak on abort

This issue surfaced in twacube.test after the previous patches.

* spot/twaalgos/ltl2tgba_fm.cc: Release the formula namer on abort.
* NEWS: Mention the bug.

NEWS

@@ -11,6 +11,10 @@ New in spot 2.11.2.dev  (not yet released)
     They have been limited to 16 operands, but this value can be changed
     with option -x tls-max-ops=N.  (Issue #521 too.)
 
+  - Running ltl_to_tgba_fm() with an output_aborter (which is done
+    during automata-based implication checks) would leak memory on
+    abort.
+
 New in spot 2.11.2  (2022-10-26)
 
   Command-line tools:

spot/twaalgos/ltl2tgba_fm.cc

@@ -2031,6 +2031,7 @@ namespace spot
       {
         if (aborter && aborter->too_large(a))
           {
+            a->release_formula_namer(namer, false);
             if (!simplifier)
               delete s;
             return nullptr;