1.1.8.5. osx.cocoa.AtSync (ObjC)
Check for nil pointers used as mutexes for @synchronized.

Examples:

void test(id x) {
  if (!x)
    @synchronized(x) {} // warn: nil value used as mutex
}

void test() {
  id y;
  @synchronized(y) {} // warn: uninitialized value used as mutex
}