don't assume *alloc functions always succeed

[halayli]

https://github.com/facebook/folly/blob/master/folly/Malloc.h#L175

https://github.com/facebook/folly/blob/master/folly/FBString.h#L304

https://github.com/facebook/folly/blob/master/folly/Malloc.h#L165

https://github.com/facebook/folly/blob/master/folly/Malloc.h#L181 -> this might return NULL and you'll lose whatever pointer it gets assigned to.

[tudor]

fixed in 050c1c3