io: more guards against NULL filename (#7159)

References ac055d677a
References #4370

src/nvim/main.c

@@ -1825,6 +1825,7 @@ static int process_env(char *env, bool is_viminit)
 /// os_fileinfo_link() respectively for extra security.
 static bool file_owned(const char *fname)
 {
+  assert(fname != NULL);
   uid_t uid = getuid();
   FileInfo file_info;
   bool file_owned = os_fileinfo(fname, &file_info)

src/nvim/memfile.c

@@ -895,6 +895,7 @@ static bool mf_do_open(memfile_T *mfp, char_u *fname, int flags)
 {
   // fname cannot be NameBuff, because it must have been allocated.
   mf_set_fnames(mfp, fname);
+  assert(mfp->mf_fname != NULL);
 
   /// Extra security check: When creating a swap file it really shouldn't
   /// exist yet. If there is a symbolic link, this is most likely an attack.

src/nvim/memline.c

@@ -1460,6 +1460,7 @@ static int process_still_running;
  */
 static time_t swapfile_info(char_u *fname)
 {
+  assert(fname != NULL);
   int fd;
   struct block0 b0;
   time_t x = (time_t)0;
@@ -3135,6 +3136,7 @@ attention_message (
     char_u *fname         /* swap file name */
 )
 {
+  assert(buf->b_fname != NULL);
   time_t x, sx;
   char        *p;
 

src/nvim/os/fs.c

@@ -859,8 +859,11 @@ bool os_fileinfo(const char *path, FileInfo *file_info)
 /// @param[out] file_info Pointer to a FileInfo to put the information in.
 /// @return `true` on success, `false` for failure.
 bool os_fileinfo_link(const char *path, FileInfo *file_info)
-  FUNC_ATTR_NONNULL_ALL
+  FUNC_ATTR_NONNULL_ARG(2)
 {
+  if (path == NULL) {
+    return false;
+  }
   uv_fs_t request;
   int result = uv_fs_lstat(&fs_loop, &request, path, NULL);
   file_info->stat = request.statbuf;