segmentation fault on `watch` testcase

[yjh0502]

Here's a log

https://gist.github.com/yjh0502/9e4f9f7376edf1e3c3e2c657c996a2bb

rustc version: rustc 1.25.0 (84203cac6 2018-03-25)
foundationdb: 5.1.5-1 (matches with CI version)

[yjh0502]

Here's a valgrind log with debug symbol. I built a fdbclient with debug symbol.
foudationdb version: 5.1.7 (9ad8d02386d4a6a5efecf898df80f2747695c627)

https://gist.github.com/yjh0502/ae0efed18c056f9a526e924bc34bea5b