stupidgcm: set dummy locking callback.

In general, OpenSSL is only threadsafe if you provide a locking function
through CRYPTO_set_locking_callback. However, the GCM operations that
stupidgcm uses never call that function.

To guard against that ever changing, set a dummy locking callback
that crashes the app.

internal/stupidgcm/locking.go

@@ -0,0 +1,26 @@
+package stupidgcm
+
+// In general, OpenSSL is only threadsafe if you provide a locking function
+// through CRYPTO_set_locking_callback. However, the GCM operations that
+// stupidgcm uses never call that function. Additionally, the manual locking
+// has been removed completely in openssl 1.1.0.
+
+/*
+#include <openssl/evp.h>
+#include <stdio.h>
+
+static void dummy_callback(int mode, int n, const char *file, int line) {
+	printf("stupidgcm: thread locking is not implemented and should not be "
+		"needed. Please upgrade openssl.\n");
+	// panic
+	*((int*)0)=0;
+}
+static void set_dummy_callback() {
+	CRYPTO_set_locking_callback(dummy_callback);
+}
+*/
+import "C"
+
+func init() {
+	C.set_dummy_callback()
+}