When a pool resource is constructed with max_blocks_per_chunk=1 it ends
up creating a pool with blocks_per_chunk=0 which means it never
allocates anything. Instead it returns null pointers, which should be
impossible.
To avoid this problem, round the max_blocks_per_chunk value to a
multiple of four, so it's never smaller than four.
libstdc++-v3/ChangeLog:
PR libstdc++/94160
* src/c++17/memory_resource.cc (munge_options): Round
max_blocks_per_chunk to a multiple of four.
(__pool_resource::_M_alloc_pools()): Simplify slightly.
* testsuite/20_util/unsynchronized_pool_resource/allocate.cc:
Check that valid pointers are returned when small values are
used for max_blocks_per_chunk.
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