kgdb/kdb: A fix for kdb command table expansion

When locally adding in some additional kdb commands, I stumbled across an issue with the dynamic expansion of the kdb command table. When the number of kdb commands exceeds the size of the statically allocated kdb_base_commands[] array, additional space is allocated in the kdb_register_repeat() routine.

The unused portion of the newly allocated array was not being initialized to zero properly and this would result in segfaults when help '?' was executed or when a search for a non-existing command would traverse the command table beyond the end of valid command entries and then attempt to use the non-zeroed area as actual command entries.

Index: b/kernel/debug/kdb/kdb_main.c
===================================================================
@@ -3341,7 +3341,7 @@ int kdb_register_repeat(char *cmd,
 			  (kdb_max_commands - KDB_BASE_CMD_MAX) * sizeof(*new));
 			kfree(kdb_commands);
 		}
-		memset(new + kdb_max_commands, 0,
+		memset(new + kdb_max_commands - KDB_BASE_CMD_MAX, 0,
 		       kdb_command_extend * sizeof(*new));
 		kdb_commands = new;
 		kp = kdb_commands + kdb_max_commands - KDB_BASE_CMD_MAX;