Use FMT_NULL instead of 0 is a few more places.

Found with GCC and -Wzero-as-null-pointer-constant

include/fmt/core.h

@@ -172,7 +172,7 @@ class basic_string_view {
   typedef Char char_type;
   typedef const Char *iterator;
 
-  FMT_CONSTEXPR basic_string_view() FMT_NOEXCEPT : data_(0), size_(0) {}
+  FMT_CONSTEXPR basic_string_view() FMT_NOEXCEPT : data_(FMT_NULL), size_(0) {}
 
   /** Constructs a string reference object from a C string and a size. */
   FMT_CONSTEXPR basic_string_view(const Char *s, size_t size) FMT_NOEXCEPT

include/fmt/posix.cc

@@ -71,7 +71,7 @@ fmt::BufferedFile::~BufferedFile() FMT_NOEXCEPT {
 
 fmt::BufferedFile::BufferedFile(
     fmt::cstring_view filename, fmt::cstring_view mode) {
-  FMT_RETRY_VAL(file_, FMT_SYSTEM(fopen(filename.c_str(), mode.c_str())), 0);
+  FMT_RETRY_VAL(file_, FMT_SYSTEM(fopen(filename.c_str(), mode.c_str())), FMT_NULL);
   if (!file_)
     FMT_THROW(system_error(errno, "cannot open file {}", filename.c_str()));
 }

include/fmt/posix.h

@@ -134,7 +134,7 @@ class BufferedFile {
 
  public:
   // Constructs a BufferedFile object which doesn't represent any file.
-  BufferedFile() FMT_NOEXCEPT : file_(0) {}
+  BufferedFile() FMT_NOEXCEPT : file_(FMT_NULL) {}
 
   // Destroys the object closing the file it represents if any.
   FMT_API ~BufferedFile() FMT_DTOR_NOEXCEPT;