patch 8.2.1972: crash when recreating nested fold

Problem:    Crash when recreating nested fold.
Solution:   Check for empty growarray. (closes #7278)

Author: brammool

src/fold.c

@@ -640,7 +640,10 @@ foldCreate(linenr_T start, linenr_T end)
 		break;
 	    }
 	}
-	i = (int)(fp - (fold_T *)gap->ga_data);
+	if (gap->ga_len == 0)
+	    i = 0;
+	else
+	    i = (int)(fp - (fold_T *)gap->ga_data);
     }
 
     if (ga_grow(gap, 1) == OK)

src/testdir/test_fold.vim

@@ -844,4 +844,14 @@ func Test_move_no_folds()
   bwipe!
 endfunc
 
+" this was crashing
+func Test_fold_create_delete_create()
+  new
+  fold
+  fold
+  normal zd
+  fold
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1972,
 /**/
     1971,
 /**/