AP_Mission: correct off-by-one check when fetching mission items

libraries/AP_Mission/AP_Mission.cpp

@@ -429,7 +429,7 @@ bool AP_Mission::set_current_cmd(uint16_t index)
 bool AP_Mission::read_cmd_from_storage(uint16_t index, Mission_Command& cmd) const
 {
     // exit immediately if index is beyond last command but we always let cmd #0 (i.e. home) be read
-    if (index > (unsigned)_cmd_total && index != 0) {
+    if (index >= (unsigned)_cmd_total && index != 0) {
         return false;
     }
 

libraries/AP_Mission/AP_Mission.h

@@ -294,6 +294,7 @@ public:
     mission_state state() const { return _flags.state; }
 
     /// num_commands - returns total number of commands in the mission
+    ///                 this number includes offset 0, the home location
     uint16_t num_commands() const { return _cmd_total; }
 
     /// num_commands_max - returns maximum number of commands that can be stored