ok_to_export() (su.c) and export() (login.c) read s[n] before
strncmp(s, *pp, n) confirms s is at least n bytes long, where n is
the length of the blocked variable name. A PAM environment entry
shorter than that name over-reads past its heap allocation.
Check strncmp() first; it stops at s's NUL terminator and never
reads past the boundary that guarded the s[n] access.
Sponsored by: Advanced Micro Devices, Inc.
Signed-off-by: Osvaldo Janeri Filho <ojanerif@amd.com>