Switch to using memcmp instead of strncmp

I introduced strncmp in http://https://github.com/bazelbuild/bazel/commit/feca25daffe5f294dfdf407bb924190d8116b012. The hand-rolled cookie logic should be replaced and conversations with the blaze team have started around that.

PiperOrigin-RevId: 233458738
diff --git a/src/main/cpp/blaze.cc b/src/main/cpp/blaze.cc
index 98592a0..e38a5a0 100644
--- a/src/main/cpp/blaze.cc
+++ b/src/main/cpp/blaze.cc
@@ -1591,7 +1591,7 @@
   if (cookie_length != cookieB.size()) {
     return false;
   }
-  return strncmp(cookieA.c_str(), cookieB.c_str(), cookie_length) == 0;
+  return memcmp(cookieA.c_str(), cookieB.c_str(), cookie_length) == 0;
 }
 
 GrpcBlazeServer::GrpcBlazeServer(int connect_timeout_secs) {