treewide: properly clear the buffer

treewide: properly clear the buffer

Before this patch, the OSD would repeat the last buffer
content in case the new buffer content would be empty.

This was mostly happening for the `title` OSD field that is intended
to be empty when it matches the app_id / WM_CLASS of the application.
Due to only buffer.len being reset but its internal allocations being
untouched, buffer.buf would still carry the old data.

This patch fixes it by also overwriting the first byte in the buffer
allocation with '\0' via the new `buf_clear()` function.

Do the same for buf_expand_shell_variables() although that one should
have been fine before as it always writes new data to the buffer.