src: remove superfluous check in backtrace_posix.cc

The error check doesn't matter because a failure would be ignored
as part of the loop condition.

PR-URL: https://github.com/nodejs/node/pull/16950


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>