Fixed segfault on uems->default is NULL

[swuecho]

the format of my change is not quite right, I try to get it right, but failed.

when uems->defaults is NULL, mongo::fromjson will segfault.

BTW, what does the defaults in uems->defaults mean? thanks.

[unbit]

Honestly i do not remember what defaults is, maybe @WeirdCarrotMonster could help ?

[WeirdCarrotMonster]

uems->defaults provide default values for emperor attributes. I should have set them to empty dict by default and add few checks for value type.

[swuecho]

Thanks. Could you take a look at my pull request or find a better fix? @WeirdCarrotMonster

[xrmx]

@swuecho Does this patch works for you? xrmx@132a90c

[swuecho]

@xrmx Should work. Thanks.

[xrmx]

@swuecho Fixed in git master 71d6572