Vulkan: log dlerror on error

Previously, it prints dlerror even when it succeeds, which leads to a
misleading log entry.

Bug: n/a
Test: cvd start -gpu_mode=guest_swiftshader
Change-Id: I6eaf9df6b3fc5c848c999847d712391e6d8b1633
diff --git a/vulkan/libvulkan/driver.cpp b/vulkan/libvulkan/driver.cpp
index aea897c..0b5bb15 100644
--- a/vulkan/libvulkan/driver.cpp
+++ b/vulkan/libvulkan/driver.cpp
@@ -183,8 +183,12 @@
                 .library_namespace = library_namespace,
             };
             so = android_dlopen_ext(lib_name.c_str(), LIB_DL_FLAGS, &dlextinfo);
-            ALOGE("Could not load %s from updatable gfx driver namespace: %s.",
-                  lib_name.c_str(), dlerror());
+            if (!so) {
+                ALOGE(
+                    "Could not load %s from updatable gfx driver namespace: "
+                    "%s.",
+                    lib_name.c_str(), dlerror());
+            }
         } else {
             // load built-in driver
             so = android_load_sphal_library(lib_name.c_str(), LIB_DL_FLAGS);