[subprocess] Address autoinit errors.

Enabling -fauto-var-init exposed errors due to not initializing locals, ZX-3970.

This fixes the tests, but also initializes all variables here to prevent
future bugs.

It also fixes a possible bug where we try to use a command read from the
channel even if the channel read failed.

Test: runtests -t {job-test, process-test} now pass.
Change-Id: I3fcdb46d0d22400519615de781d57e6c41a460e6