Possible dangling `else` with DLOG in non-debug mode

After #830, the following code starts misbehaving:
```cpp
#define NDEBUG
#include "glog/logging.h"

int main()
{
  if (true)
    DLOG(ERROR) << "Hello";
  else
    LOG(ERROR) << "Bye";
}
```

This code now compiles with a warning, and prints "Bye".

Another possible issue with #830 is that `DLOG` now expands to a statement in non-debug mode, while being an expression in debug mode.