Fix an issue introduced by check-in [4cd2a9672c59] (2017-03-03) that could

Fix an issue introduced by check-in [4cd2a9672c59] (2017-03-03) that could
allow a negative value in the 3rd parameter to memmove() when
defragmentPage() is called on a btree page with a corrupted
freeblock list.  The corruption is now detected early and results in
an SQLITE_CORRUPT return before the memmove() is reached.

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