Check setegid() return values

The clang static analyzer is complaining that we are not checking the
setegid() return value, so let's add these checks.  We should never
fail to return to AID_ROOT, so fail hard in this case.

Bug: 26962034
Test: Boot bullhead
Test: export WITH_STATIC_ANALYZER=1 and run 'mm' in the project directory
Change-Id: I62e95b045c5734305c71502871b6cf17f152edbc