initialize variable in tests

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.

src/tests.c

@@ -4324,8 +4324,10 @@ void test_ecdsa_sign_verify(void) {
     secp256k1_scalar one;
     secp256k1_scalar msg, key;
     secp256k1_scalar sigr, sigs;
-    int recid;
     int getrec;
+    /* Initialize recid to suppress a false positive -Wconditional-uninitialized in clang.
+       VG_UNDEF ensures that valgrind will still treat the variable as uninitialized. */
+    int recid = -1; VG_UNDEF(&recid, sizeof(recid));
     random_scalar_order_test(&msg);
     random_scalar_order_test(&key);
     secp256k1_ecmult_gen(&ctx->ecmult_gen_ctx, &pubj, &key);