This PR fixes an issue that may sporadically trigger ASAN to got into a
deadlock when running a subprocess through the `IO.Process.spawn`
framework.
The general issue here is that we run `fork()` and then perform an
allocation in the child before going to `execvp` (for allocating the
arguments to `execvp`). As it turns out, doing this can cause a race
condition in ASAN that ultimately causes a deadlock in the child. This
was fixed upstream but then rolled back (see
https://github.com/google/sanitizers/issues/774). Thus, we must avoid
allocating any memory in between `fork` and `execvp`.
c9facc8102