src: fix node_report maybe-uninitialized warning

g++ 8.2 reports warning:

outstream may be used uninitialized in this function [-Wmaybe-uninitialized]

It's wrong, but initialize the variable anyhow to silence the warning.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines