refactor: suppress clang and PVS warnings (#19940)

src/nvim/eval.c

@@ -8148,6 +8148,7 @@ repeat:
         // Only replace it when it starts with '~'
         if (*dirname == '~') {
           s = xstrdup(dirname);
+          assert(s != NULL);  // suppress clang "Argument with 'nonnull' attribute passed null"
           *fnamep = s;
           xfree(*bufp);
           *bufp = s;

src/nvim/eval/typval.c

@@ -1252,7 +1252,7 @@ static void do_sort_uniq(typval_T *argvars, typval_T *rettv, bool sort)
           idx++;
           li = TV_LIST_ITEM_NEXT(l, li);
         }
-        if (info.item_compare_func_err) {
+        if (info.item_compare_func_err) {  // -V547
           emsg(_("E882: Uniq compare function failed"));
           break;
         }

src/nvim/eval/vars.c

@@ -791,6 +791,7 @@ static void ex_unletlock(exarg_T *eap, char *argstart, int deep, ex_unletlock_ca
         semsg(_(e_invarg2), arg - 1);
         return;
       }
+      assert(*lv.ll_name == '$');  // suppress clang "Uninitialized argument value"
       if (!error && !eap->skip && callback(&lv, arg, eap, deep) == FAIL) {
         error = true;
       }

src/nvim/insexpand.c

@@ -640,6 +640,7 @@ static char_u *ins_compl_infercase_gettext(char_u *str, int char_len, int compl_
   while (i < char_len) {
     if (gap.ga_data != NULL) {
       ga_grow(&gap, 10);
+      assert(gap.ga_data != NULL);  // suppress clang "Dereference of NULL pointer"
       p = (char *)gap.ga_data + gap.ga_len;
       gap.ga_len += utf_char2bytes(wca[i++], p);
     } else if ((p - (char *)IObuff) + 6 >= IOSIZE) {