Fixed a small logical mistake in zathura/utils.c
The function file_valid_extension should return True if plugin is different then NULL (i.e. there exists a plugin to handle the file extension, which is therefore valid), not if it is equal.
The function file_valid_extension should return True if plugin is different then NULL (i.e. there exists a plugin to handle the file extension, which is therefore valid), not if it is equal.