fix(NVIM_APPNAME): show error message if $NVIM_APPNAME is invalid

Closes https://github.com/neovim/neovim/issues/23056.

src/nvim/main.c

@@ -250,6 +250,11 @@ int main(int argc, char **argv)
 
   argv0 = argv[0];
 
+  if (!appname_is_valid()) {
+    os_errmsg("$NVIM_APPNAME is not a valid file name.\n");
+    exit(1);
+  }
+
   if (argc > 1 && STRICMP(argv[1], "-ll") == 0) {
     if (argc == 2) {
       print_mainerr(err_arg_missing, argv[1]);

src/nvim/os/stdpaths.c

@@ -69,6 +69,19 @@ const char *get_appname(void)
   return env_val;
 }
 
+/// Ensure that APPNAME is valid. In particular, it cannot contain directory separators.
+bool appname_is_valid(void)
+{
+  const char *appname = get_appname();
+  const size_t appname_len = strlen(appname);
+  for (size_t i = 0; i < appname_len; i++) {
+    if (appname[i] == PATHSEP) {
+      return false;
+    }
+  }
+  return true;
+}
+
 /// Return XDG variable value
 ///
 /// @param[in]  idx  XDG variable to use.