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;