[RFC] spatch to drop check after alloc/lalloc/xmalloc

Sample of what can be done with [coccinelle](http://coccinelle.lip6.fr/) for helping resolving tickets like #488 . This patch built by:
````
spatch --in-place --sp-file allocs.cocci src/*.c
````
When contents of `allocs.cocci` (that's the whole contents):
````patch
@@ identifier p; statement S; @@
  p =
(
  alloc(...)
|
  lalloc(...)
|
  xmalloc(...)
)
  ;
- if (p == NULL) S
````

In case of situations like:
````c
    s1 = alloc(newlen);
    if (s1 == NULL)                 /* out of memory */
      return FAIL;
    s2 = alloc(newlen);
````
It generates extra space in place of removed statement.

And for
````c
      vim_free(arshape_buf);
      buflen = len * 2 + 2;
      arshape_buf = alloc(buflen);
      if (arshape_buf == NULL)
        return;         /* out of memory */
````
It leaves comment in place of removed content since comment isn't catched by statement metavariable.