address compiler warnings gcc-13

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/nlsat/nlsat_explain.cpp

@@ -126,7 +126,7 @@ namespace nlsat {
         scoped_literal_vector   m_core2;
 
         // temporary fields for storing the result
-        scoped_literal_vector * m_result;
+        scoped_literal_vector * m_result = nullptr;
         svector<char>           m_already_added_literal;
 
         evaluator &             m_evaluator;
@@ -149,8 +149,6 @@ namespace nlsat {
             m_todo(u),
             m_core1(s),
             m_core2(s),
-            m_result(nullptr),
-
             m_cell_sample(is_sample),
 
             m_evaluator(ev) {

src/nlsat/nlsat_simplify.cpp

@@ -93,7 +93,6 @@ namespace nlsat {
         }
 
         void update_clauses(u_map<literal> const& b2l) {
-            bool is_sat = true;
             literal_vector lits;
             unsigned n = m_clauses.size();