Don't free aliased pointers in ctx cmp_ctx tests

Don't free aliased pointers in ctx cmp_ctx tests

Coverity recorded issues 1551739 and 1551737, a potential double free in the
tests.  It occurs when the DUP operation fails in such a way val3_read is
returned as the same pointer as val2_read.  Ideally it should never
happen, but resetting val3_read to 0 should satisfy coverity that there
is no issue here

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/22800)