patch 9.0.0873: using freed memory when executing mapclear at more prompt

Problem:    Using freed memory when executing mapclear at the more prompt.
Solution:   Do not clear mappings while listing them. (closes #11438)

src/map.c

@@ -24,6 +24,10 @@ static mapblock_T	*first_abbr = NULL; // first entry in abbrlist
 static mapblock_T	*(maphash[256]);
 static int		maphash_valid = FALSE;
 
+// When non-zero then no mappings can be added or removed.  Prevents mappings
+// to change while listing them.
+static int		map_locked = 0;
+
 /*
  * Make a hash value for a mapping.
  * "mode" is the lower 4 bits of the State for the mapping.
@@ -150,11 +154,15 @@ showmap(
     if (message_filtered(mp->m_keys) && message_filtered(mp->m_str))
 	return;
 
+    // Prevent mappings to be cleared while at the more prompt.
+    // Must jump to "theend" instead of returning.
+    ++map_locked;
+
     if (msg_didout || msg_silent != 0)
     {
 	msg_putchar('\n');
 	if (got_int)	    // 'q' typed at MORE prompt
-	    return;
+	    goto theend;
     }
 
     mapchars = map_mode_to_chars(mp->m_mode);
@@ -200,6 +208,9 @@ showmap(
 #endif
     msg_clr_eos();
     out_flush();			// show one line at a time
+
+theend:
+    --map_locked;
 }
 
     static int
@@ -298,6 +309,9 @@ list_mappings(
 	int	mode,
 	int	*did_local)
 {
+    // Prevent mappings to be cleared while at the more prompt.
+    ++map_locked;
+
     if (p_verbose > 0 && keyround == 1 && seenModifyOtherKeys)
 	msg_puts(_("Seen modifyOtherKeys: true"));
 
@@ -337,6 +351,8 @@ list_mappings(
 	    }
 	}
     }
+
+    --map_locked;
 }
 
 /*
@@ -954,6 +970,21 @@ map_clear(
     map_clear_mode(curbuf, mode, local, abbr);
 }
 
+/*
+ * If "map_locked" is set then give an error and return TRUE.
+ * Otherwise return FALSE.
+ */
+    static int
+is_map_locked(void)
+{
+    if (map_locked > 0)
+    {
+	emsg(_(e_cannot_change_mappings_while_listing));
+	return TRUE;
+    }
+    return FALSE;
+}
+
 /*
  * Clear all mappings in "mode".
  */
@@ -968,6 +999,9 @@ map_clear_mode(
     int		hash;
     int		new_hash;
 
+    if (is_map_locked())
+	return;
+
     validate_maphash();
 
     for (hash = 0; hash < 256; ++hash)