Fix potential NULL dereference

Fixes Coverity CID 1496225
diff --git a/lib/openssl.c b/lib/openssl.c
index d907346..92358e1 100644
--- a/lib/openssl.c
+++ b/lib/openssl.c
@@ -229,7 +229,7 @@
 
     ret = 0;
 Exit:
-    if (ret != 0) {
+    if (ret != 0 && *ctx != NULL) {
         x9_62_free_context(*ctx);
         *ctx = NULL;
     }