FFI: Make FFI call return nil when C function returns NULL Before when a C function returned NULL pointer, an ExternalAddress with address 0 wes returned which required an extra check in a caller to convert it to nil, which would be the customary. This commit automatically converts NULL pointers to Smalltalk `nil` as a courtesy to a Smalltalker.

LIBRARY libbasic
DESCRIPTION     'Smalltalk/X Basic classes'
VERSION         3.5
CODE            EXECUTE READ 
DATA            READ WRITE
SECTIONS
    INITCODE    READ EXECUTE 
    INITDATA    READ WRITE 
EXPORTS 
    _libbasic_Init