Fix false positive coverity report

C++ standard:
-8- An object whose initializer is an empty set of parentheses, i.e.,
   (), shall be value-initialized.

-5- To value-initialize an object of type T means:
   -- if T is a non-union class type without a user-provided
      constructor, then every non-static data member and base-class
      component of T is value-initialized;93)

   93) Value-initialization for such a class object may be
       implemented by zero-initializing the object and then calling
       the default constructor.

   -- if T is an array type, then each element is value-initialized;

   -- otherwise, the object is zero-initialized

hence, m_fmt is zero initialised by the default constructor.

And regardless, m_fmt would have been set during AVFormatWriter::Init() which is always called.

Coverity-Id: 609723