coverity 1456640: fix null check

Reviewed-by: NTim Hudson <tjh@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10748)

providers/implementations/serializers/serializer_dh_priv.c

@@ -48,11 +48,10 @@ static void *dh_priv_newctx(void *provctx)
 
     if (ctx != NULL) {
         ctx->provctx = provctx;
-    }
-
-    /* -1 is the "whatever" indicator, i.e. the PKCS8 library default PBE */
-    ctx->sc.pbe_nid = -1;
 
+        /* -1 is the "whatever" indicator, i.e. the PKCS8 library default PBE */
+        ctx->sc.pbe_nid = -1;
+    }
     return ctx;
 }