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/math/simplex/bit_matrix.cpp

@@ -125,7 +125,7 @@ unsigned_vector bit_matrix::gray(unsigned n) {
         auto v = gray(n-1);
         auto w = v;
         w.reverse();
-        for (auto & u : v) u |= (1 << (n-1));
+        for (auto & u : v) u |= (1u << (n-1));
         v.append(w);
         return v;
     }