[mojo] Pass the full error message length to the process error callback

Change-Id: I9ad0f3e030a95055a93b2a567317f513af641505
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2076328Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745571}

mojo/public/cpp/system/invitation.cc

@@ -46,7 +46,7 @@ void RunErrorCallback(uintptr_t context,
   std::string error_message;
   if (details->error_message) {
     error_message =
-        std::string(details->error_message, details->error_message_length - 1);
+        std::string(details->error_message, details->error_message_length);
     callback->Run(error_message);
   } else if (details->flags & MOJO_PROCESS_ERROR_FLAG_DISCONNECTED) {
     delete callback;