Fix an infinite loop in the MEMSYS5 auxiliary memory allocator that occurs

Fix an infinite loop in the MEMSYS5 auxiliary memory allocator that occurs
for memory allocations between 1GiB and 2GiB in size.  Error introduced
by check-in [949133231f8f751a].  The problem only affects builds that
include the SQLITE_ENABLE_MEMSYS5 compile-time option.

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