Fix segfaults when varbind cannot be constructed

For multiple reasons, `varbind` can be a null pointer, which causes segmentation faults.

With this change, the value is checked and the segmentation fault avoided.

Co-authored-by: kain88-de <[email protected]>
Co-authored-by: spt29 <[email protected]>

Author: 3 people

python/netsnmp/client_intf.c

@@ -1520,6 +1520,10 @@ netsnmp_get_or_getnext(PyObject *self, PyObject *args, int pdu_type,
           continue;
 
       varbind = PySequence_GetItem(varlist, varlist_ind);
+      if (varbind == NULL) {
+        /* PySequence_GetItem failed - break to avoid NULL deref in build_python_varbind */
+        break;
+      }
       type = build_python_varbind(varbind, vars, varlist_ind, sprintval_flag,
                                   &len, &str_buf, getlabel_flag);
       if (type != TYPE_OTHER) {
@@ -1828,6 +1832,9 @@ netsnmp_walk(PyObject *self, PyObject *args)
               }
 
               varbind = py_netsnmp_construct_varbind();
+              if (varbind == NULL)
+                break;
+
               if (varbind && build_python_varbind(varbind, vars, varlist_ind,
                                        sprintval_flag, &len, &str_buf, getlabel_flag) !=
                   TYPE_OTHER) {
@@ -2051,6 +2058,9 @@ netsnmp_getbulk(PyObject *self, PyObject *args)
 	    vars = vars->next_variable, varbind_ind++) {
 
 	  varbind = py_netsnmp_construct_varbind();
+	      if (varbind == NULL)
+	        break;
+
           if (varbind && build_python_varbind(varbind, vars, varbind_ind,
                               sprintval_flag, &len, &str_buf, getlabel_flag) != TYPE_OTHER) {
             const int hex = is_hex(str_buf, len);