bpo-41056: Fix a NULL pointer dereference on MemoryError within the ssl module. (GH-21009)

Detected by Coverity.
(cherry picked from commit eb0d5c3)

Co-authored-by: Gregory P. Smith <[email protected]>

Author: miss-islington

Misc/NEWS.d/next/Library/2020-06-20-18-35-43.bpo-41056.Garcle.rst

@@ -0,0 +1 @@
+Fix a NULL pointer dereference within the ssl module during a MemoryError in the keylog callback. (discovered by Coverity)

Modules/_ssl/debughelpers.c

@@ -125,6 +125,12 @@ _PySSL_keylog_callback(const SSL *ssl, const char *line)
 
     threadstate = PyGILState_Ensure();
 
+    ssl_obj = (PySSLSocket *)SSL_get_app_data(ssl);
+    assert(PySSLSocket_Check(ssl_obj));
+    if (ssl_obj->ctx->keylog_bio == NULL) {
+        return;
+    }
+
     /* Allocate a static lock to synchronize writes to keylog file.
      * The lock is neither released on exit nor on fork(). The lock is
      * also shared between all SSLContexts although contexts may write to
@@ -141,12 +147,6 @@ _PySSL_keylog_callback(const SSL *ssl, const char *line)
         }
     }
 
-    ssl_obj = (PySSLSocket *)SSL_get_app_data(ssl);
-    assert(PySSLSocket_Check(ssl_obj));
-    if (ssl_obj->ctx->keylog_bio == NULL) {
-        return;
-    }
-
     PySSL_BEGIN_ALLOW_THREADS
     PyThread_acquire_lock(lock, 1);
     res = BIO_printf(ssl_obj->ctx->keylog_bio, "%s\n", line);