When you read from a default-constructed MPMCQueue, assert instead of SIGFPE.

When you read from a default-constructed MPMCQueue, assert instead of SIGFPE.

Summary:
I accidentally forgot to specify the capacity for my
MPMCQueue.  When I then did a blockingRead(), I got a SIGFPE.  Thanks
to a custom signal handler that doesn't print stack traces, and a few
more comedy of errors, I lost a day to this.  With this patch, I would
have gotten an assertion failure instead.

Reviewed By: yfeldblum

Differential Revision: D4169033

fbshipit-source-id: fab97ea0d5afc3c06885758b31a5e8c91ae75a45