snapuserd: Fix ubsan when reading a single aligned sector.

When a read request is for a single sector, and the sector is
block-aligned, it falls into the ReadAlignedSector path, which assumes
block-sized reads. Fix this by clamping the expected size.

Bug: 291862304
Test: manual test of ReadWorker::ReadAlignedSector
      full OTA
Change-Id: I00e460c333e8a9a4dc2433443e3633f3d794da1d