Use NULL comparison

Reviewed-by: Richard Levitte <levitte@openssl.org>
diff --git a/crypto/engine/tb_eckey.c b/crypto/engine/tb_eckey.c
index a1cffe8..dbb4139 100644
--- a/crypto/engine/tb_eckey.c
+++ b/crypto/engine/tb_eckey.c
@@ -76,7 +76,7 @@
 
 int ENGINE_register_EC(ENGINE *e)
 {
-    if (e->ec_meth)
+    if (e->ec_meth != NULL)
         return engine_table_register(&dh_table,
                                      engine_unregister_all_EC, e, &dummy_nid,
                                      1, 0);
@@ -93,7 +93,7 @@
 
 int ENGINE_set_default_EC(ENGINE *e)
 {
-    if (e->ec_meth)
+    if (e->ec_meth != NULL)
         return engine_table_register(&dh_table,
                                      engine_unregister_all_EC, e, &dummy_nid,
                                      1, 1);