UnixProcess_md.c: avoid potential deadlock after vfork am: c88b39b4d1

Original change: https://googleplex-android-review.googlesource.com/c/platform/libcore/+/18184955

Change-Id: If1d0ff30c7f1093527b7b12c7f3f85127df44228
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>