Simplify BYTE_CODE_SAFE checking

Simplify BYTE_CODE_SAFE checking

* src/bytecode.c (BYTE_CODE_SAFE): Default to false, so that it
can be used outside #if.  All uses of ‘defined BYTE_CODE_SAFE’
changed to ‘BYTE_CODE_SAFE’.  Use BYTE_CODE_SAFE in plain
expressions instead of #if expressions when this is easy.
(struct byte_stack) [BYTE_CODE_SAFE]: Remove member ‘bottom’,
as it is no longer needed.
(exec_byte_code): Omit #if BYTE_CODE_SAFE when this is easy.
Simplify stack-overflow checking when BYTE_CODE_SAFE.