Clear p->fvalue when handling a try/catch

p->fvalue will be set by BIFs that generate exceptions
(such as throw/1), and it will not be cleared before another
exception is generated. Potentially, p->fvalue may contain a
huge term (e.g. after throw(HugeTerm)) which will be kept in
the heap.

We can shorten the lifetime of f->value by clearing it in
the instructions that handle catches: catch_end and try_end.
That is safe because BEAM code will never access p->fvalue.
If BEAM code needs to rethrow an exception it will use a
reference to the value passed in an X register.

The reason that p->fvalue must not be cleared already in
handle_error() is that native code trap handlers will use
it. (See the comment before handle_error() and the comment
at the end of handle_error() in beam_emu.c for some more
information about exception handling.)

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