_get_impl segfaults

Sometimes, functions/methods calling get or getMulti segfault. if I run make test a few times I can get it to fail at least once.

Here's the backtrace:

(gdb) bt
#0 0x0000000107b6c1cb in _zval_dtor_func ()
#1 0x0000000107ba3adf in ZEND_ASSIGN_SPEC_CV_CONST_HANDLER ()
#2 0x0000000107bd0b5d in execute ()
#3 0x0000000107b631da in zend_call_function ()
#4 0x000000010854ca68 in php_couchbase_get_impl () at couchbase.c:829
#5 0x0000000107bd0480 in zend_do_fcall_common_helper_SPEC ()
#6 0x0000000107bd0b5d in execute ()
#7 0x0000000107b6c6d9 in zend_execute_scripts ()
#8 0x0000000107b1d03e in php_execute_script ()
#9 0x0000000107be6cd8 in main ()