analyzer: fix build error with clang (PR 93543)

analyzer: fix build error with clang (PR 93543)

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=243681 reports a build
failure with clang 9.0.1:

gcc/analyzer/engine.cc:2971:13: error:
      reinterpret_cast from 'nullptr_t' to 'function *' is not allowed
  v.m_fun = reinterpret_cast<function *> (NULL);
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
engine.cc:2983:21: error:
      reinterpret_cast from 'nullptr_t' to 'function *' is not allowed
  return v.m_fun == reinterpret_cast<function *> (NULL);
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The casts appears to be unnecessary; eliminate them.

gcc/analyzer/ChangeLog:
	PR analyzer/93543
	* engine.cc (pod_hash_traits<function_call_string>::mark_empty):
	Eliminate reinterpret_cast.
	(pod_hash_traits<function_call_string>::is_empty): Likewise.