Add explicit null checks to some stdio functions.

Applications fopening files and then blindly trying to read are
widespread, leading to a recurring problem of SELinux tightening
resulting in segfaults. Add a friendly diagnostic for this case.

Bug: http://b/67455242
Test: bionic-unit-tests32/64 on sailfish
Change-Id: I1734fa94487c4eff9b55a02c6b01baf6b265d236