Check for NULL pointers before finding the syshandle
Robert Sprowson authored
menu.c: if NULL return -1, this has the added advantage that a menu maker (see event_attachmenumaker) can return NULL to mean "no menu thanks" and not data abort in event.c
dbox.c: if NULL return 0, this is somewhat consistent with template_syshandle()
menu.h: typo in comment corrected

Version 5.82. Tagged as 'RISC_OSLib-5_82'
da9f2a03