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)
diff --git a/src/mbyte.c b/src/mbyte.c
index 5ed321e..575fd52 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -4666,8 +4666,7 @@
 	else if (ICONV_ERRNO != ICONV_E2BIG)
 	{
 	    /* conversion failed */
-	    vim_free(result);
-	    result = NULL;
+	    VIM_CLEAR(result);
 	    break;
 	}
 	/* Not enough room or skipping illegal sequence. */