NULL ptr deref in initial_state_start_fun

[Steve Grubb]

I spent some time fuzzing this library until I got a crash. The crash is at
lib/metalink_pstate.c line 103. This is called by lib/libexpat_metalink_parser.c at line 81. The issue is that if "name" does not have NAMESPACE_SEPARATOR, then split_ns_name leaves ns_uri == NULL. The fix is to check ns_uri != NULL before using it in initial_state_start_fun at lines 103 and 119.