Validate LDDW second-half instruction fields

tests/errors/err-lddw-second-half-invalid.data

@@ -0,0 +1,6 @@
+-- raw
+0x0000000000000018
+0x0000000000000100
+0x0000000000000095
+-- error
+Failed to load code: invalid lddw second half at PC 1

vm/ubpf_vm.c

@@ -1868,6 +1868,10 @@ validate(const struct ubpf_vm* vm, const struct ebpf_inst* insts, uint32_t num_i
                 *errmsg = ubpf_error("incomplete lddw at PC %d", i);
                 return false;
             }
+            if (insts[i + 1].dst != 0 || insts[i + 1].src != 0 || insts[i + 1].offset != 0) {
+                *errmsg = ubpf_error("invalid lddw second half at PC %d", i + 1);
+                return false;
+            }
             i++; /* Skip next instruction */
             break;