Replace alloc_check by xmalloc

`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

src/memory.c

@@ -72,22 +72,6 @@ char_u *alloc_clear(unsigned size)
   return (char_u *)xcalloc(1, (size_t)size);
 }
 
-/*
- * alloc() with check for maximum line length
- */
-char_u *alloc_check(unsigned size)
-{
-#if !defined(UNIX) && !defined(__EMX__)
-  if (sizeof(int) == 2 && size > 0x7fff) {
-    /* Don't hide this message */
-    emsg_silent = 0;
-    EMSG(_("E340: Line is becoming too long"));
-    return NULL;
-  }
-#endif
-  return lalloc((long_u)size, TRUE);
-}
-
 /*
  * Allocate memory like lalloc() and set all bytes to zero.
  */