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/util/sorting_network.h

@@ -412,7 +412,7 @@ Notes:
                 bits++;
                 w_max >>= 1;
             }
-            unsigned pow = (1ul << (bits-1));
+            unsigned pow = bits > 0 ? (1u << (bits-1)) : 0;
             unsigned a = (k + pow - 1) / pow; // a*pow >= k
             SASSERT(a*pow >= k);
             SASSERT((a-1)*pow < k);