[Support] Fix UB in BumpPtrAllocator when first allocation is zero.

BumpPtrAllocator::Allocate() is marked __attribute__((returns_nonnull)) when the
compiler supports it, which makes it UB to return null.

When there have been no allocations yet, the current slab is [nullptr, nullptr).
A zero-sized allocation fits in this range, and so Allocate(0, 1) returns null.

There's no explicit docs whether Allocate(0) is valid. I think we have to assume
that it is:
 - the implementation tries to support it (e.g. >= tests instead of >)
 - malloc(0) is allowed
 - requiring each callsite to do a check is bug-prone
 - I found real LLVM code that makes zero-sized allocations

Differential Revision: https://reviews.llvm.org/D125040