Limit the size of SrcList objects to 200 entries (compile-time configurable

Limit the size of SrcList objects to 200 entries (compile-time configurable
using -DSQLITE_MAX_SRCLIST=n).  The maximum number of tables in a join has
always been 64, so this is not a real constraint on capability.  Limiting the
size of a SrcList prevents DOS attacks (discovered by OSSFuzz) using crazy
nexted CTE joins.

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