util: Avoid output duplication in IndentedStream

Change-Id: Ia530878068da89a1839574c9c5f1dc5d1de8b54e
Fixes: #2723
diff --git a/src/util/indented-stream.cpp b/src/util/indented-stream.cpp
index 590a7ce..f6f559d 100644
--- a/src/util/indented-stream.cpp
+++ b/src/util/indented-stream.cpp
@@ -62,6 +62,7 @@
   for (const StringView& line : splitOutput) {
     m_output << m_indent << line << "\n";
   }
+  str(std::string());
   return 0; // success
 }