-client only works for the first instance of Qt Creator

• Open 2 instances of Creator
• Close the first one
• Execute qtcreator -client foo.cpp
• A new instance is started instead of opening in the previous one. This is true even if -pid is specified.

firstPeer is locked by the first instance.

When the second instance is started, it the file is already locked, so it can't lock it again. When qtcreator -client is started, isClient() returns false because the file is not locked anymore, so a new instance is started.