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backtrace_eval_unrewind is used to temporarily reverse
let-bindings (it's called with a positive argument to reverse
bindings, and then a negative argument to re-apply them) by
backtrace--locals and backtrace-eval.  For the SPECPDL_LET_DEFAULT and
SPECPDL_LET_LOCAL cases (which occur for let-bindings on buffer-local
variables), the code calls Fdefault_value and Fbuffer_local_value on
the symbol.

For symbols which are unbound at top-level, the first (with positive
argument) call to backtrace_eval_unrewind will set the symbol's value
to unbound (putting the current value in the specpdl's "old value"
slot).  On the second (with negative argument) call,
backtrace_eval_unrewind attempts to retrieve the symbol's value with
Fdefault_value or Fbuffer_local_value, but that raises a void-variable
signal.  This interrupts the restoration of the let-bindings, so any
other variables more recent on the stack will now have the wrong
value.

* src/data.c (default_value): Make non-static.
* src/lisp.h: Declare it.
* src/eval.c (backtrace_eval_unrewind): Replace the calls to
Fdefault_value and Fbuffer_local_value with default_value and
buffer_local_value, respectively.  The latter do exactly the same as
the former, except if the symbol's value is Qunbound they just return
it instead of signaling void-variable.