Check that the stack is initialized when an exception may occur

Strengthen beam_validator to check that the stack is initialized
when an instruction with an {f,0} operand is executed.
For example, the following code sequence:

    {allocate,0,1}.
    {bif,element,{f,0},[{integer,1},{x,0}],{x,0}}.

should not be accepted because the stack may be scanned if
element/2 fails. That could cause a crash or other undefined
behavior if garbage on the stack looks like a catch tag.

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