path: change mp_splitext() semantics

Including the "." in the returned extension was too inconvenient. I
think originally, the semantics were supposed to work like in Python,
but screw this.

Also, return NULL instead of "" on failure (which is what its only user
actually seems to expect).

options/path.c

@@ -155,11 +155,11 @@ char *mp_splitext(const char *path, bstr *root)
 {
     assert(path);
     const char *split = strrchr(path, '.');
-    if (!split)
-        split = path + strlen(path);
+    if (!split || !split[1] || strchr(split, '/'))
+        return NULL;
     if (root)
-        *root = (bstr){.start = (char *)path, .len = path - split};
-    return (char *)split;
+        *root = (bstr){(char *)path, split - path};
+    return (char *)split + 1;
 }
 
 char *mp_path_join(void *talloc_ctx, struct bstr p1, struct bstr p2)

options/path.h

@@ -50,10 +50,10 @@ char *mp_get_user_path(void *talloc_ctx, struct mpv_global *global,
 
 char *mp_basename(const char *path);
 
-/* Return file extension, including the '.'. If root is not NULL, set it to the
- * part of the path without extension. So: path == root + returnvalue
+/* Return file extension, excluding the '.'. If root is not NULL, set it to the
+ * part of the path without extension. So: path == root + "." + extension
  * Don't consider it a file extension if the only '.' is the first character.
- * Return "" if no extension.
+ * Return NULL if no extension and don't set *root in this case.
  */
 char *mp_splitext(const char *path, bstr *root);
 

player/screenshot.c

@@ -347,7 +347,7 @@ void screenshot_to_file(struct MPContext *mpctx, const char *filename, int mode,
     }
     char *ext = mp_splitext(filename, NULL);
     if (ext)
-        opts.format = ext + 1; // omit '.'
+        opts.format = ext;
     struct mp_image *image = screenshot_get(mpctx, mode);
     if (!image) {
         screenshot_msg(ctx, SMSG_ERR, "Taking screenshot failed.");