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).

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.");