Add compatibility impl for EVP_PKEY_get0

Node.js uses EVP_PKEY_get0, which is present in OpenSSL but which
BoringSSL currently does not export. This CL adds an implementation
for it, which Electron is currently floating as a patch.

See
https://github.com/nodejs/node/commit/6a7eb32c5bcd92f490a57f2cd12d52e1db881d17
from Node.

Change-Id: I2474cacbd22882355a8037e2033739f7496b21f2
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/47824
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

crypto/evp/evp.c

diff --git a/crypto/evp/evp.c b/crypto/evp/evp.c
index 653d657..bb31645 100644
--- a/crypto/evp/evp.c
+++ b/crypto/evp/evp.c
@@ -429,6 +429,15 @@
                            0, (void *)out_md);
 }
 
+void *EVP_PKEY_get0(const EVP_PKEY *pkey) {
+  // Node references, but never calls this function, so for now we return NULL.
+  // If other projects require complete support, call |EVP_PKEY_get0_RSA|, etc.,
+  // rather than reading |pkey->pkey.ptr| directly. This avoids problems if our
+  // internal representation does not match the type the caller expects from
+  // OpenSSL.
+  return NULL;
+}
+
 void OpenSSL_add_all_algorithms(void) {}
 
 void OPENSSL_add_all_algorithms_conf(void) {}

include/openssl/evp.h

diff --git a/include/openssl/evp.h b/include/openssl/evp.h
index 378eb32..405b402 100644
--- a/include/openssl/evp.h
+++ b/include/openssl/evp.h
@@ -836,6 +836,11 @@
 // Ed448 and attempts to create keys will fail.
 #define EVP_PKEY_ED448 NID_ED448
 
+// EVP_PKEY_get0 returns NULL. This function is provided for compatibility with
+// OpenSSL but does not return anything. Use the typed |EVP_PKEY_get0_*|
+// functions instead.
+OPENSSL_EXPORT void *EVP_PKEY_get0(const EVP_PKEY *pkey);
+
 // OpenSSL_add_all_algorithms does nothing.
 OPENSSL_EXPORT void OpenSSL_add_all_algorithms(void);