patch 9.0.1391: "clear" macros are not always used

Problem:    "clear" macros are not always used.
Solution:   Use ALLOC_ONE, VIM_CLEAR, CLEAR_POINTER and CLEAR_FIELD in more
            places. (Yegappan Lakshmanan, closes #12104)
diff --git a/src/textprop.c b/src/textprop.c
index bead146..c6464fc 100644
--- a/src/textprop.c
+++ b/src/textprop.c
@@ -1718,8 +1718,7 @@
 			if (ii < gap->ga_len)
 			{
 			    char_u **p = ((char_u **)gap->ga_data) + ii;
-			    vim_free(*p);
-			    *p = NULL;
+			    VIM_CLEAR(*p);
 			    did_remove_text = TRUE;
 			}
 		    }