avoid invalid iterator dereference in get_allocator()

[spoonincode]

get_allocator() needs to return the bip allocator for a given pointer based on looking up that pointer in a global map of address ranges, or return nullopt_t if the pointer is not within any address range (which means it wasn't allocated in a bip allocator, such as being allocated on stack).

Consider there is a single range registered (_segment_manager_map.size() == 1) and this range is from 0x2000 to 0x4000. get_allocator(0x5000) will cause upper_bound() to return end(). Then reversing the iterator by one via *(--it) will return the 0x2000-0x4000 entry, the if (object < seg_end) check fails, and nullopt_t is returned as expected.

But get_allocator(0x1000) will return begin(). This causes the *(--it) to iterate off the valid range and dereference an invalid iterator.

Add a check for begin() and immediately return nullopt_t.

[heifner]

Consider adding a test case.

[spoonincode]

Consider adding a test case.

Fortunately, the existing shared_vector_apis_segment_alloc test was running afoul here