fix --prof-process --preprocess flag

This is a one-line fix to prevent the --preprocess
option (used with --prof-process to output JSON)
to cause an isolate log file profiling process to crash.

PR-URL: https://github.com/nodejs/node/pull/14966


Reviewed-By: Luca Maraschi <luca.maraschi@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>