Work around spurious g++-12 warnings

* spot/twaalgos/ltl2tgba_fm.cc, spot/tl/formula.hh,
spot/twaalgos/translate.cc: Add SPOT_ASSUME in various places to help
g++.

spot/tl/formula.hh

@@ -294,7 +294,9 @@ namespace spot
       {
         if (SPOT_UNLIKELY(i >= size()))
           report_non_existing_child();
-        return children[i];
+        const fnode* c = children[i];
+        SPOT_ASSUME(c != nullptr);
+        return c;
       }
 
       /// \see formula::ff

spot/twaalgos/ltl2tgba_fm.cc

@@ -1026,11 +1026,16 @@ namespace spot
           bool coacc = false;
           auto& st = sm->states_of(n);
           for (auto l: st)
-            if (namer->get_name(l).accepts_eword())
+            {
+              formula lf = namer->get_name(l);
+              // Somehow gcc 12.2.0 thinks lf can be nullptr.
+              SPOT_ASSUME(lf != nullptr);
+              if (lf.accepts_eword())
                 {
                   coacc = true;
                   break;
                 }
+            }
           if (!coacc)
             {
               // ... or if any of its successors is coaccessible.

spot/twaalgos/translate.cc

@@ -210,6 +210,9 @@ namespace spot
           {
             auto safety = [](formula f)
             {
+              // Prevent gcc 12.2.0 from warning us that f could be a
+              // nullptr formula.
+              SPOT_ASSUME(f != nullptr);
               return f.is_syntactic_safety();
             };
             auto i = std::remove_if(oblg.begin(), oblg.end(), safety);