[nightlies] Crash on amd64 during test run

The tests are currently crashing on amd64 on Ubuntu 13.10, with this stack from valgrind:

==5691== Process terminating with default action of signal 11 (SIGSEGV): dumping core
==5691== Access not within mapped region at address 0x0
==5691== at 0xA1C1790: extract_field_values (u1db_query.c:308)
==5691== by 0xA1C1CFE: op_combine (u1db_query.c:499)
==5691== by 0xA1C2081: evaluate_index_and_insert_into_db (u1db_query.c:1570)
==5691== by 0xA1C2798: sqlite_cb_to_field_cb (u1db_query.c:1494)
==5691== by 0x9B37EBF: sqlite3_exec (in /usr/lib/x86_64-linux-gnu/libsqlite3.so.0.8.6)
==5691== by 0xA1C3E91: u1db__update_indexes (u1db_query.c:1512)
==5691== by 0xA1BBC50: write_doc (u1db.c:483)
==5691== by 0xA1BE778: u1db_put_doc (u1db.c:689)
==5691== by 0xA1BE8C4: u1db_create_doc_from_json (u1db.c:358)
==5691== by 0xA19DB6C: __pyx_pf_4u1db_5tests_17c_backend_wrapper_9CDatabase_18create_doc_from_json (c_backend_wrapper.c:10261)
==5691== by 0xA19D97E: __pyx_pw_4u1db_5tests_17c_backend_wrapper_9CDatabase_19create_doc_from_json (c_backend_wrapper.c:10180)
==5691== by 0x487578: PyCFunction_Call (methodobject.c:85)
==5691== If you believe this happened as a result of a stack
==5691== overflow in your program's main thread (unlikely but
==5691== possible), you can try to increase the size of the
==5691== main thread stack using the --main-stacksize= flag.
==5691== The main thread stack size used in this run was 16777216.
==5691==