common: Fix debug log not to generate any compiler warnings

When debug log is disabled the following code:

  DEBUG_LOG("something")

was expanded by the preprocessor to

  if (false) *static_cast<std::ofstream*>(nullptr);

Compiling such a code was generating a compiler warning about statement
which has no effect. This commit modifies the macro definition to avoid
this problem by changing it to:

  if (false) *static_cast<std::ofstream*>(nullptr) << 0;

This will also work with expressions like:

  DEBUG_LOG("something") << xxx;

which will be seen as:

  if (false) *static_cast<std::ofstream*>(nullptr) << 0 << xxx;

Change-Id: I8ce935b4b025bc53dd1e2794b7cd30217b739e09