fixed a signed/unsigned error (found by -Werror)

git-svn-id: svn://bind10.isc.org/svn/bind10/trunk@1298 e5f2f494-b856-4b98-b285-d166d9295462

src/lib/dns/rrttl.cc

@@ -34,7 +34,7 @@ RRTTL::RRTTL(const string& ttlstr)
     // Some systems (at least gcc-4.4) flow negative values over into
     // unsigned integer, where older systems failed to parse. We want
     // that failure here, so we extract into int64 and check the value
-    uint64_t val;
+    int64_t val;
 
     istringstream iss(ttlstr);
     iss >> dec >> val;