[committed] correct strcmp() == 0 result for unknown strings (PR 92157)

[Martin Sebor]

The optimization to fold (strcmp() == 0) results involving
arrays/strings of unequal size/length has a bug where it is
unprepared for the compute_string_length() function to return
an invalid length as an indication that the length is unknown.
This leads to some strings that are unequal being considered
equal.

The attached patch corrects this handling.  I have committed
it in r277194 with Jeff's okay.

Martin

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