Fix static analysis findings: uninitialized vars, bitwise shift UB, garbage values

- nla_core.cpp: Initialize j = null_lpvar in is_octagon_term
- bit2int.cpp: Initialize sign_p, sign_n, sz_p, sz_n
- act_cache.cpp: Initialize debug vars to nullptr
- enum2bv_rewriter.cpp: Use unsigned literal in 1u << idx
- bit_matrix.cpp: Use unsigned literal in 1u << (n-1)
- bit_util.cpp: Guard against bit_shift == 0 in shl/shr
- mpff.cpp: Cast exp to unsigned before shifting
- sorting_network.h: Guard against bits == 0
- dl_sparse_table.h: Use >= 64 instead of == 64

Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

src/ast/act_cache.cpp

@@ -173,7 +173,7 @@ void act_cache::insert(expr * k, unsigned offset, expr * v) {
         DEBUG_CODE(expected_tag = 0;); 
     }
     DEBUG_CODE({
-        expr * v2;
+        expr * v2 = nullptr;
         SASSERT(m_table.find(e, v2));
         SASSERT(v == UNTAG(expr*, v2));
         SASSERT(expected_tag == GET_TAG(v2));
@@ -195,7 +195,7 @@ expr * act_cache::find(expr * k, unsigned offset) {
         SASSERT(m_unused > 0);
         m_unused--;
         DEBUG_CODE({
-            expr * v;
+            expr * v = nullptr;
             SASSERT(m_table.find(e, v));
             SASSERT(GET_TAG(v) == 1);
         });

src/ast/rewriter/bit2int.cpp

@@ -354,8 +354,8 @@ void bit2int::visit(app* n) {
         //
         // (pos1 - neg1) mod e2 = (pos1 + (e2 - (neg1 mod e2))) mod e2
         //
-        unsigned sz_p, sz_n, sz;
-        bool sign_p, sign_n;
+        unsigned sz_p = 0, sz_n = 0, sz;
+        bool sign_p = false, sign_n = false;
         expr_ref tmp_p(m), tmp_n(m);
         VERIFY(extract_bv(pos1, sz_p, sign_p, tmp_p));
         VERIFY(extract_bv(neg1, sz_n, sign_n, tmp_n));

src/ast/rewriter/enum2bv_rewriter.cpp

@@ -64,7 +64,7 @@ struct enum2bv_rewriter::imp {
             unsigned bv_size = get_bv_size(s);
             sort_ref bv_sort(m_bv.mk_sort(bv_size), m);
             if (is_unate(s))
-                return m_bv.mk_numeral(rational((1 << idx) - 1), bv_sort.get());
+                return m_bv.mk_numeral(rational((1u << idx) - 1), bv_sort.get());
             else 
                 return m_bv.mk_numeral(rational(idx), bv_sort.get());
         }