Add an assertion to the fixed-size allocator to ensure that we don't request
an allocation that is greater than what we will actually allocate.

Patch by Artyom Skrobov!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189340 91177308-0d34-0410-b5e6-96231b3b80d8