Make a hard copy of strings in constraint checks prior to applying

Make a hard copy of strings in constraint checks prior to applying
OP_RealAffinity, to avoid problems with a pointer accounting assert.
This change is not strictly necessary - the correct answer is obtained
without it and no UB occurs - however the pointer accounting asserts are
useful to prevent other problems so it is a simple matter to bring this
piece into compliance.  Ticket [5ad2aa6921faa1ee]

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