DrmStatus: refactor string ctor

Bug: 249941041
Test: MediaDrmParameterizedTests
Change-Id: Id87719324c06f99ee16807e73e4d42f40eb83275
diff --git a/drm/libmediadrm/interface/mediadrm/DrmUtils.h b/drm/libmediadrm/interface/mediadrm/DrmUtils.h
index 3028b78..2632ebd 100644
--- a/drm/libmediadrm/interface/mediadrm/DrmUtils.h
+++ b/drm/libmediadrm/interface/mediadrm/DrmUtils.h
@@ -289,7 +289,9 @@
 template<typename T>
 std::string GetExceptionMessage(const DrmStatus &err, const char *defaultMsg, const sp<T> &iface) {
     Vector<::V1_4::LogMessage> logs;
-    iface->getLogMessages(logs);
+    if (iface != NULL) {
+        iface->getLogMessages(logs);
+    }
     return GetExceptionMessage(err, defaultMsg, logs);
 }