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()