Fix asan build

Summary:
The array-bounds checker in gcc complains that r could be -1 after leaving CHECK_ERR in ASAN mode. Switch to DCHECK to avoid this possibility. The code was not initially compiling; now it does.

Error from gcc:
  array subscript is below array bounds [-Werror=array-bounds]

Reviewed By: yfeldblum

Differential Revision: D3232694

fb-gh-sync-id: 17f579eb152b661a8e5e4ed29ef91eec405fb90a
fbshipit-source-id: 17f579eb152b661a8e5e4ed29ef91eec405fb90a

folly/test/FileLockTest.cpp

@@ -43,7 +43,7 @@ TEST(File, Locks) {
   static constexpr size_t pathLength = 2048;
   char buf[pathLength + 1];
   int r = readlink("/proc/self/exe", buf, pathLength);
-  CHECK_ERR(r);
+  CHECK(r != -1);
   buf[r] = '\0';
 
   fs::path me(buf);