patch 8.0.1496: clearing a pointer takes two lines

Problem:    Clearing a pointer takes two lines.
Solution:   Add VIM_CLEAR() and replace vim_clear(). (Hirohito Higashi,
            closes #2629)

src/misc2.c

@@ -1827,6 +1827,8 @@ copy_option_part(
  * Replacement for free() that ignores NULL pointers.
  * Also skip free() when exiting for sure, this helps when we caught a deadly
  * signal that was caused by a crash in free().
+ * If you want to set NULL after calling this function, you should use
+ * VIM_CLEAR() instead.
  */
     void
 vim_free(void *x)
@@ -1840,19 +1842,6 @@ vim_free(void *x)
     }
 }
 
-/*
- * Like vim_free(), and also set the pointer to NULL.
- */
-    void
-vim_clear(void **x)
-{
-    if (*x != NULL)
-    {
-	vim_free(*x);
-	*x = NULL;
-    }
-}
-
 #ifndef HAVE_MEMSET
     void *
 vim_memset(void *ptr, int c, size_t size)