extensions BUGFIX plugins refcount maintenance

increase refcount only in case the context being created is successfully
created and returned from ly_ctx_new().
diff --git a/src/context.c b/src/context.c
index 49ab2c2..4305907 100644
--- a/src/context.c
+++ b/src/context.c
@@ -78,7 +78,6 @@
     lydict_init(&ctx->dict);
 
     /* plugins */
-    ext_plugins_ref++;
     lyext_load_plugins();
 
     /* models list */
@@ -88,6 +87,7 @@
         free(ctx);
         return NULL;
     }
+    ext_plugins_ref++;
     ctx->models.used = 0;
     ctx->models.size = 16;
     if (search_dir) {