Fix clang syntax warning.

The error message is
In file included from ../Core/CoreDatabase.cpp:30:
../Core/CoreDatabase.h:1545:26: error: read-only variable is not assignable
               *pRefCats = -1;
               ~~~~~~~~~ ^
std::set is an ordered data structure, directly change one of the element
may destroy the order, it's dangerous.

BUG=chromium:219559
TEST=CFLAGS="-clang -print-cmdline" CXXFLAGS="-clang -print-cmdline"
     emerge-x86-alex gobi3k-sdk passes.

Change-Id: I4e5f0a01cc920e81e8a3484e85111b1a00d3c7f4
Reviewed-on: https://gerrit.chromium.org/gerrit/47675
Reviewed-by: Han Shen <shenhan@chromium.org>
Reviewed-by: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
diff --git a/Core/CoreDatabase.h b/Core/CoreDatabase.h
index c4c47e2..25ba606 100644
--- a/Core/CoreDatabase.h
+++ b/Core/CoreDatabase.h
@@ -1549,7 +1549,8 @@
 
                log.Log( tmp.str(), eDB2_STATUS_ERROR );
 
-               *pRefCats = -1;
+               cats.erase(pRefCats);
+               cats.insert(-1);
                bRC = false;
             }
          }