If capture-output is requested but one of the channels goes unused (eg.
we attempt to capture stderr but the command never writes to stderr), we
can leak memory.

guest_exec_output_watch() is (from what I understand) unconditionally
called for both streams if output capture is requested. The first call
will always pass the `p->size == p->length` check b/c both values are
0. Then GUEST_EXEC_IO_SIZE bytes will be allocated for the stream.

But when we reap the exited process there's a `gei->err.length > 0`
check to actually free the buffer. Which does not get run if the command
doesn't write to the stream.

Fix by making free() unconditional.

Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>