misc/path_utils: fix some bugs

The detection for leading dots was in the wrong place, in case path was
not already a basename. Fixed by acting on basename. Also all leading
dots are now getting skipped, otherwise the root of `path/to/...ext`
could end up as `path/to/..` which in the wrong place/context would mean
directory traversal.

Additionally, since we act on the basename there is no longer a need to
test for trailing path separators. The previous check also did not
account for Windows paths with \ separators.

Author: WhitePeter

misc/path_utils.c

@@ -85,9 +85,15 @@ void mp_path_strip_trailing_separator(char *path)
 char *mp_splitext(const char *path, bstr *root)
 {
     mp_assert(path);
-    int skip = (*path == '.'); // skip leading dot for "hidden" unix files
-    const char *split = strrchr(path + skip, '.');
-    if (!split || !split[1] || strchr(split, '/'))
+    char *bn = mp_basename(path);
+
+    // Skip all leading dots, not just for "hidden" unix files, otherwise we
+    // end up splitting a part of the filename sans leading dot.
+    while (*bn == '.')
+        bn++;
+
+    const char *split = strrchr(bn, '.');
+    if (!split || !split[1])
         return NULL;
     if (root)
         *root = (bstr){(char *)path, split - path};