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
diff --git a/tests/fortify_compilation_test.cpp b/tests/fortify_compilation_test.cpp
index d859ef1..7517fde 100644
--- a/tests/fortify_compilation_test.cpp
+++ b/tests/fortify_compilation_test.cpp
@@ -376,6 +376,7 @@
   // This is fine.
   realpath(".", NULL);
 
-  // FIXME: But we should warn on this.
-  realpath(NULL, buf);
+  char bigbuf[PATH_MAX];
+  // CLANG: error: 'realpath': NULL path is never correct; flipped arguments?
+  realpath(NULL, bigbuf);
 }