AP_Mission: add sanity check that 16-bit commands aren't stored_in_location

This is a rather confusing problem when it happens during development,
so a sanity check is good.  What you see in dev is that your latitude
gets corrupted between when you store and retrieve it, with no warnings
that you're doing something wrong.

libraries/AP_Mission/AP_Mission.cpp

@@ -538,6 +538,14 @@ bool AP_Mission::read_cmd_from_storage(uint16_t index, Mission_Command& cmd) con
     }
 
     if (stored_in_location(cmd.id)) {
+#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
+        // NOTE!  no 16-bit command may be stored_in_location as only
+        // 10 bytes are available for storage and lat/lon/alt required
+        // 4*sizeof(float) == 12 bytes of storage.
+        if (b1 == 0) {
+            AP_HAL::panic("May not store location for 16-bit commands");
+        }
+#endif
         // Location is not PACKED; field-wise copy it:
         cmd.content.location.relative_alt = packed_content.location.flags.relative_alt;
         cmd.content.location.loiter_ccw = packed_content.location.flags.loiter_ccw;