That appears to be a segmentation fault, which would be a PHP crash/bug as a PHP app shouldn't be able to trigger that directly. (Except maybe for runaway recursion, but I don't think that would be the case here.)
You may need to get a core dump and then process it with gdb to see if it gives some useful information. See:
https://rtcamp.com/tutorials/php/core-dump-php5-fpm/