Don't ignore child process output if it exits with -1 exit code.

Don't ignore child process output if it exits with -1 exit code.

While this code is used by us if the program couldn't be launched at all, it
doesn't mean that it didn't run as -1 could also be returned by the child
process to indicate an error after outputting something, so we should still
read its output in this case.

Closes #15205.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74352 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775