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/mark.c b/src/mark.c
index 342bfcd..1acdd12 100644
--- a/src/mark.c
+++ b/src/mark.c
@@ -127,8 +127,7 @@
 	    i = c - 'A';
 	namedfm[i].fmark.mark = *pos;
 	namedfm[i].fmark.fnum = fnum;
-	vim_free(namedfm[i].fname);
-	namedfm[i].fname = NULL;
+	VIM_CLEAR(namedfm[i].fname);
 #ifdef FEAT_VIMINFO
 	namedfm[i].time_set = vim_time();
 #endif
@@ -598,8 +597,7 @@
 	    && fnamecmp(name, fm->fname) == 0)
     {
 	fm->fmark.fnum = buf->b_fnum;
-	vim_free(fm->fname);
-	fm->fname = NULL;
+	VIM_CLEAR(fm->fname);
     }
 }
 
@@ -862,8 +860,7 @@
 			else
 			    n = i - 'A';
 			namedfm[n].fmark.mark.lnum = 0;
-			vim_free(namedfm[n].fname);
-			namedfm[n].fname = NULL;
+			VIM_CLEAR(namedfm[n].fname);
 #ifdef FEAT_VIMINFO
 			namedfm[n].time_set = 0;
 #endif
@@ -1480,16 +1477,14 @@
     {
 	for (i = 0; i < NMARKS + EXTRA_MARKS; ++i)
 	    vim_free(vi_namedfm[i].fname);
-	vim_free(vi_namedfm);
-	vi_namedfm = NULL;
+	VIM_CLEAR(vi_namedfm);
     }
 #ifdef FEAT_JUMPLIST
     if (vi_jumplist != NULL)
     {
 	for (i = 0; i < vi_jumplist_len; ++i)
 	    vim_free(vi_jumplist[i].fname);
-	vim_free(vi_jumplist);
-	vi_jumplist = NULL;
+	VIM_CLEAR(vi_jumplist);
     }
 #endif
 }