Fix use-of-uninitialized-value with exception on deprecated const access

[iluuu1994]

Caught by various tests in nightly. https://github.com/php/php-src/actions/runs/14767850964/job/41462688558

The use-of-uninitialized-value happens on this line:

php-src/Zend/zend_vm_def.h

Line 8192 in 178fc2d

zval_ptr_dtor_nogc(EX_VAR(throw_op->result.var));

It happens because because the ZEND_HANDLE_EXCEPTION handler assumes the result has already been initialized and potentially needs to be freed. To fix this, simply initialize the result to NULL, like we already do a few lines above.