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If grow_specpdl fails due to outgrowing max_specpdl_size, it will
signal an error *before* growing the specpdl array.  Therefore, when
handling the signal, specpdl_ptr points past the end of the specpdl
array and any further use of of specpdl before unwinding (e.g., if
edebug binds signal-hook-function) will cause memory corruption.
* src/eval.c (signal_or_quit): Don't call `signal-hook-function' if
the specpdl_ptr is already past the end of the specpdl array.
* test/src/eval-tests.el (eval-tests--exceed-specbind-limit)
(eval-exceed-specbind-with-signal-hook): New test & helper function.