[secp256k1] initialize variable in tests
[secp256k1] initialize variable in tests

Summary:
> This was detected while running the tests with the `-Wconditional-uninitialized` flag
>
> ```
> ./autogen.sh
> CC=clang CFLAGS="-Wconditional-uninitialized" ./configure
> make check
> ```
>
> The resulting warning is a false positive, but setting the value to -1
> ensures that the CHECK below will fail if recid is never written to.

This is a backport of [[https://github.com/bitcoin-core/secp256k1/pull/889 | secp256k1#889]]

It should allow us to finally land D9110

Test Plan:
To check that this diff does not break existing tests:
`ninja && ninja check-secp256k1`

To check that this removes the compilation warning: rebased D9110 on top of this

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9372

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