gcc/analyzer: fix false positives for NULL (#13248)

Close https://github.com/neovim/neovim/issues/13158

src/nvim/api/vim.c

@@ -2687,6 +2687,9 @@ void nvim__screenshot(String path)
 
 static void clear_provider(DecorProvider *p)
 {
+  if (p == NULL) {
+    return;
+  }
   NLUA_CLEAR_REF(p->redraw_start);
   NLUA_CLEAR_REF(p->redraw_buf);
   NLUA_CLEAR_REF(p->redraw_win);

src/nvim/eval/encode.c

@@ -159,8 +159,11 @@ static int conv_error(const char *const msg, const MPConvStack *const mpstack,
           vim_snprintf((char *)IObuff, IOSIZE, idx_msg, idx);
           ga_concat(&msg_ga, IObuff);
         } else {
-          typval_T key_tv = *TV_LIST_ITEM_TV(
-              tv_list_first(TV_LIST_ITEM_TV(li)->vval.v_list));
+          assert(li != NULL);
+          listitem_T *const first_item =
+            tv_list_first(TV_LIST_ITEM_TV(li)->vval.v_list);
+          assert(first_item != NULL);
+          typval_T key_tv = *TV_LIST_ITEM_TV(first_item);
           char *const key = encode_tv2echo(&key_tv, NULL);
           vim_snprintf((char *) IObuff, IOSIZE, key_pair_msg, key, idx);
           xfree(key);

src/nvim/ex_cmds.c

@@ -3451,13 +3451,13 @@ static buf_T *do_sub(exarg_T *eap, proftime_T timeout,
 
   sub_firstline = NULL;
 
-  /*
-   * ~ in the substitute pattern is replaced with the old pattern.
-   * We do it here once to avoid it to be replaced over and over again.
-   * But don't do it when it starts with "\=", then it's an expression.
-   */
-  if (!(sub[0] == '\\' && sub[1] == '='))
+  // ~ in the substitute pattern is replaced with the old pattern.
+  // We do it here once to avoid it to be replaced over and over again.
+  // But don't do it when it starts with "\=", then it's an expression.
+  assert(sub != NULL);
+  if (!(sub[0] == '\\' && sub[1] == '=')) {
     sub = regtilde(sub, p_magic);
+  }
 
   // Check for a match on each line.
   // If preview: limit to max('cmdwinheight', viewport).

src/nvim/extmark.c

@@ -74,6 +74,7 @@ uint64_t extmark_set(buf_T *buf, uint64_t ns_id, uint64_t id,
                      Decoration *decor, ExtmarkOp op)
 {
   ExtmarkNs *ns = buf_ns_ref(buf, ns_id, true);
+  assert(ns != NULL);
   mtpos_t old_pos;
   uint64_t mark = 0;