libc fortify: error on realpath(NULL, foo)

I've half a mind to make this a warning instead, since this sort of
call isn't UB. That said:
- if the user really wants this (I can't imagine why they would), they
  can just put NULL in a non-const variable,
- we're slowly moving to -Werror ~everywhere anyway, and
- it's presumably easier to change this from an error to a warning than
  the other way around

Bug: 12231437
Test: m checkbuild on bullhead internal master. No new
CtsBionicTestCases failures.

Change-Id: Ie8bf5a3455f663686fda4a7450fb35d147fa745e