SDL_enabled_assert(): Use `NULL` istead of `0` to explicity initialize the pointer members of SDL_AssertData

(cherry picked from commit 272b0733237555c749bc6dd82a8dd0211ab2e22d)

include/SDL_assert.h

@@ -153,7 +153,7 @@ extern DECLSPEC SDL_AssertState SDLCALL SDL_ReportAssertion(SDL_AssertData *,
 #define SDL_enabled_assert(condition) \
     do { \
         while ( !(condition) ) { \
-            static struct SDL_AssertData sdl_assert_data = { 0, 0, #condition, 0, 0, 0, 0 }; \
+            static struct SDL_AssertData sdl_assert_data = { 0, 0, #condition, NULL, 0, NULL, NULL }; \
             const SDL_AssertState sdl_assert_state = SDL_ReportAssertion(&sdl_assert_data, SDL_FUNCTION, SDL_FILE, SDL_LINE); \
             if (sdl_assert_state == SDL_ASSERTION_RETRY) { \
                 continue; /* go again. */ \