patch 9.0.0949: crash when unletting a variable while listing variables

Problem:    Crash when unletting a variable while listing variables.
Solution:   Disallow changing a hashtable while going over the entries.
            (closes #11435)

src/structs.h

@@ -1313,6 +1313,12 @@ typedef struct hashitem_S
 // This allows for storing 10 items (2/3 of 16) before a resize is needed.
 #define HT_INIT_SIZE 16
 
+// flags used for ht_flags
+#define HTFLAGS_ERROR	0x01	// Set when growing failed, can't add more
+				// items before growing works.
+#define HTFLAGS_FROZEN	0x02	// Trying to add or remove an item will result
+				// in an error message.
+
 typedef struct hashtable_S
 {
     long_u	ht_mask;	// mask used for hash value (nr of items in
@@ -1321,8 +1327,7 @@ typedef struct hashtable_S
     long_u	ht_filled;	// number of items used + removed
     int		ht_changed;	// incremented when adding or removing an item
     int		ht_locked;	// counter for hash_lock()
-    int		ht_error;	// when set growing failed, can't add more
-				// items before growing works
+    int		ht_flags;	// HTFLAGS_ values
     hashitem_T	*ht_array;	// points to the array, allocated when it's
 				// not "ht_smallarray"
     hashitem_T	ht_smallarray[HT_INIT_SIZE];   // initial array