selftests/sgx: Fix uninitialized pointer dereference in error path

Ensure ctx is zero-initialized, such that the encl_measure function will
not call EVP_MD_CTX_destroy with an uninitialized ctx pointer in case of an
early error during key generation.

Fixes: 2adcba79e69d ("selftests/x86: Add a selftest for SGX")
Signed-off-by: Jo Van Bulck <[e-mail]>
Signed-off-by: Dave Hansen <[e-mail]>
Reviewed-by: Jarkko Sakkinen <[e-mail]>
Acked-by: Kai Huang <[e-mail]>
Link: https://lore.kernel.org/all/20231005153854.25566-2-jo.vanbulck%40cs.kuleuven.be

tools/testing/selftests/sgx/sigstruct.c

···
 	struct sgx_sigstruct *sigstruct = &encl->sigstruct;
 	struct sgx_sigstruct_payload payload;
 	uint8_t digest[SHA256_DIGEST_LENGTH];
+	EVP_MD_CTX *ctx = NULL;
 	unsigned int siglen;
 	RSA *key = NULL;
-	EVP_MD_CTX *ctx;
 	int i;
 
 	memset(sigstruct, 0, sizeof(*sigstruct));
···
 	return true;
 
 err:
-	EVP_MD_CTX_destroy(ctx);
+	if (ctx)
+		EVP_MD_CTX_destroy(ctx);
 	RSA_free(key);
 	return false;
 }