Guard against integer overflow on the DiplomaticSRAM size.

A "size=(1<<31)" returns a negative integer size (and errors out in FIRRTL),
while size=(1<<32) wraps around back to 1 (instead of getting a 4B entry RAM).

Since comically large SRAMs aren't desirable anyways (and difficult to
simulate), let us place assertion guards on the caller side of the RAM creation.