Unfortunately there's still a case where NULL can be returned from
file_pat_to_reg_pat().
xmemdupz() and xmallocz() aren't static anymore. There are many use cases for
these function.
`alloc_check` is just a wrapper around xmalloc, so we can remove it and use
xmalloc directly. ref #487 / #488
The call was replaced in the following files:
- ex_cmds.c
- misc1.c
- ops.c
