screenshot: fix %f corner case

URLs can end with trailing slashes (/) which in turn results in GNU
basename[1] returning the empty string. Catch that case and fallback on
the raw value of mpctx->filename. Subsequent path sanitation takes care
of translating invalid path component chars.

Issue reproduction steps:
```
mpv \
    --screenshot-dir=$HOME/mpv-shots \
    --screenshot-template='%f/%P' \
    https://example.org/video/
```
This would result in %f expanding to '' and thus render
screenshot-template an absolute path which, for some reason, would in
turn take precedence of screenshot-dir and hence result in a
non-writeable path, i.e. `/timestamp.ext`.

With this new approach the resulting path looks like this:
    `/home/user/mpv-shots/http:__example.org_video_/timestamp.ext`

Not particularly pretty but less ambiguous and more consistent over a
diverse range of input paths.

[1] #14635 (comment)

Author: WhitePeter

player/screenshot.c

@@ -175,12 +175,17 @@ static char *create_fname(struct MPContext *mpctx, char *template,
         }
         case 'f':
         case 'F': {
-            char *video_file = NULL;
-            if (mpctx->filename)
+            char *video_file = mpctx->filename;
+            if (!video_file) {
+                video_file = "NO_FILE";
+            } else {
                 video_file = mp_basename(mpctx->filename);
 
-            if (!video_file)
-                video_file = "NO_FILE";
+                /* This should only matter for rare URLs with a trailing /
+                 * because GNU basename then returns ''. */
+                if (!video_file[0])
+                    video_file = mpctx->filename;  // use the raw value instead
+            }
 
             char *name = video_file;
             if (fmt == 'F')
@@ -300,9 +305,6 @@ static char *gen_fname(struct mp_cmd_ctx *cmd, const char *file_ext)
             void *t = fname;
             dir = mp_get_user_path(t, ctx->mpctx->global, dir);
             fname = mp_path_join(NULL, dir, fname);
-
-            mp_mkdirp(dir);
-
             talloc_free(t);
         }