Fix #76450: SIGSEGV in firebird_stmt_execute

We need to verify that the `result_size` is not larger than our buffer,
and also should make sure that the `len` which is passed to
`isc_

Fix #76450: SIGSEGV in firebird_stmt_execute

We need to verify that the `result_size` is not larger than our buffer,
and also should make sure that the `len` which is passed to
`isc_vax_integer()` has a permissible value; otherwise we bail out.

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