Implement vim_str(n)save using xstrdup and strncpy/xmalloc
Remove NULL/non-NULL tests after calls to vim_str(n)save()
Replace alloc() with xmalloc() and remove immediate OOM checks
Remove OOM checks: reverse_text()
Remove OOM checks: mf_alloc_bhdr()
Remove OOM checks: ml_new()
Remove OOM checks: ml_new_data()
Remove OOM checks: alloc_cmdbuff()
Remove OOM checks: realloc_cmdbuff()

put_on_cmdline() doesn't FAIL anymore but its return value was
never checked.
Remove OOM checks: alloc_typebuf()
Remove OOM checks: save_typebuf()
Remove OOM checks: backslash_halve_save()
truncate_line() cant't FAIL: change its return type to void
Remove OOM checks: mark_line()
Remove OOM checks: ff_create_stack_element()
Remove OOM checks: u_save_line()
Remove OOM checks: expand_tag_fname() and tag_full_name()
Remove OOM checks: do_string_sub()
Remove OOM checks: viminfo_filename()
Remove OOM checks: suggested changes in review

 - Replace a vim_strsave/free pair with xrealloc
 - Use xmallocz() in some places
 - Use xrealloc() and forget about the NULL pointer case
 - Remove invalid comment
 - Remove unnecessary checks
 - Replace a complicated xmalloc/STRCPY/free code chunk code with xrealloc()
 - Replace a vim_strsave/free code chunk with xrealloc()